My answer from January 2014 accepts the origin date of "by 1928" for the phrase "get off [one’s] high horse" listed in Chapman & Kipfer, Dictionary of American Slang, third edition (1995) and restated as "1928+" in Kipfer & Chapman, Dictionary of American Slang, fourth edition (2007). But a search of newspaper archives yields numerous much earlier instances of related phrases and of the exact wording—and these have led me to rethink my earlier conclusion that any connection between "where do you get off?" and a person dismounting "a high horse" was extremely unlikely.
From the Launcestown [Tasmania] Advertiser (January 11, 1838):
Had the anticipations in which men gladly allowed themselves to indulge, of the results of the new reign, been realised, we should have willingly acknowledged in this call of co-operation a flattering recognition of the community in, if we may so say, its corporate capacity ; as it is, we only see a trimming to circumstances—a dismounting from the high horse, from no love of not riding horses, but from the force of an irresistible expediency. How the community, disappointed and sickened as it is, will answer to the call made upon its exertions, we shall pretend not to say.
From "Star-light," in the [Clarksville, Texas] Standard (May 20, 1848):
Well, we are not about to draw a comparison, or a contrast, between the two [the stars of heaven and the Clarksville Star newspaper], after the style of Brougham or Phillips, but we wish simply to say, that the body of the bright particular Star, the Clarksville Star, loomed above the horizon on yesterday evening, like a blazing Comet and became distinctly and luminously visible in all the rotundity of its full orbed splendor.
However, we believe we shall have to get down from our high horse, and descend to a grade of language far beneath the real merit and importance of the subject, before we shall be able to make ourself understood by common corn raising, beef eating humanity.
From "The Syracuse Doings," in the New-York Tribune (September 14, 1850):
Our City Hunkers will not be pleased that John Van Buren was allowed to rule the roast at Syracuse—that Dickinson was left uncommended—and that there are strong symptoms of his being sold out to secure the election of Seymour & Co. But they are unreasonable, and will have to come down from the high horse they have been riding for some time past and accept the Burners as colaborers and brother Democrats. This being inevitable, they can hardly set about it too soon.
From "The Transit to California," in the New York Daily Times (June 10, 1856), reprinted in the [San Francisco] Daily Alta California (July 26, 1856):
We are not sure as to the prospect of a very prompt alteration in the present schedule, but it will certainly come, and it may be, if Commodore Vanderbilt should consent to alight from his high horse and resume the practical business temper and good sense which usually characterize his management of steamboat jobs, it will be made the profit of his stockholders in the now charterless Nicaragua Transit Company.
From "Save the Union," in the Fremont [Ohio] Journal (January 1, 1858):
The South demands that the Lecompton constitution shall be adopted. Mr. Douglas and the Messenger say no.—The South says it won't stay in the Union if Mr. Douglas and the Messenger don't come down off that high horse. Mr. Douglas and the Messenger reply that they won't do it. Now we ask once mere, with trembling anxiety, what is to become of the Union?
From "Duluth," in the [Duluth, Minnesota] Labor World (September 26, 1896):
The coopers are pushing the boycott on the Imperial mill products. The committee appointed by the Trades Assembly are doing good work and the strong arm of organized labor in Duluth will yet compel Mr. B. C. Church to come off his high horse.
From a review of Rome and the Last Four Popes, in the [Sydney, New South Wales] Empire (June 16, 1858):
It [the book] is very turgid and wordy. We long to see the author got off his high horse and say what he has got, to say in a quiet manner, ^but we are never gratified.
From "Domestic Affairs of a Clergyman," in the London Globe, reprinted in the [Washington, D.C.] Evening Star (September 6, 1858):
According to the evidence of the brother, given in Court, Mr. Vansittart said he wished his wife were dead, and then "she would have no more children, which he detested." But an intimation that the Bishop's attention would be called to the case, brought him down from the high horse to a more I humble attitude, and made him supplicate for mercy. He entreated his wife to forgive him, and she did so.
From "A Battle Royal: Over the Statehood Bill Now Going On at Washington," in the [Nogales, Arizona] Oasis (January 13, 1906):
Before the session of Congress is over the "insurgents" will be glad to get down off from their high horse and vote for a rule demanded by the overwhelming majority of their party conference, Speaker Cannon will show them a few things.
And from "Chicago Street Cars and Freezing Weather," the [Chicago, Illinois] Day Book (January 6, 1912):
When the people use the power they possess as a people, even the arrogant traction interests of Chicago and all their public servants will get off their high horse and do what they could have done long ago—give the people of Chicago transportation facilities worthy of the second city in the United States of America.
It seems to me that these many instances of dismounting from a high horse (figuratively speaking) provide grounds for arguing that George Ade's use (in "The Fable of the Regular Customer and the Copper-Lined Entertainer" ) of "tell him where to get off" in the sense of "whittle him down to size" may very well allude to getting off a high horse—notwithstanding my earlier answer's conclusion to the contrary.
I haven't found any evidence of a direct connection between the two expressions, but "getting off a high horse" antedates "telling someone where to get off," which in turn antedates "where do you get off?" in the relevant sense. "After the fact, therefore because of the fact" is bad logic; but "before the fact" rules out "because of the fact" altogether—and we no longer have a situation where the idiom "tell [someone] where to get off" appears to be older than "get off [someone's] high horse."
Update (July 14, 2020)
I ran Elephind newspaper database searches for "where do you get off?" and allied phrases—and found that the question was posed in multiple instances as a question literally asking someone what location they had in mind for leaving a public conveyance. For example, from "Strangers Yet" in the Saugerties [New York] Weekly Post (April 9, 1891):
“What is the matter?”
“I am afraid to go home,” I said. I was 19 years old, but I felt like a baby.
“Where do you get off?” asked the voice.
“At St. John’s place,” I replied, and just then the conductor called out the street, and I pushed my way to the rear platform and stepped off.
From Violet Mitchell, "Miss Tilly's Bonanza," in the [Great Bend, Kansas] Barton County Democrat (May 10, 1894):
"... I don't suppose I'm much better off 'n what you be—but pore is pore —an' a man with a hole in his coat big enough for a cat to crawl through, ain't likely to be no John Jacob As tor. Now be he?"
"No," replied the man, slowly, "he isn't. Where do you get off?" he asked.
"I don't know just where I'll get off," she stammered, "that is I kalkalate to go on to Stillington, if I meet my folks there—that is—"
From "Canadian Simplicity," in the Indianapolis [Indiana] Journal (July 23, 1895):
"You didn't look at my ticket." said I, trying to detain him [the train conductor]."
"Where do you get off?" he Inquired.
"London," said I. "Why, that's a good ways off yet," sail he, smiling. "You've got one, I suppose?"
Many instances similar to these first three appear in subsequent years.
The earliest Elephind match for "Where do you get off?" that might be read as meaning "How do you have the effrontery to do or say something?" appears in "The 'Lulu' in Poker" in the Salt Lake [City, Utah] Herald (April 17, 1899), reprinted from the New York Sun:
"The what?" said the stranger with a bit of warmth in his tone. "What are you giving us? Where do you get off on this, anyhow? You've got a jack high, and mine are fours, which is a walkover," and he started to pull down the pot himself.
As suggesed in my other answer, however, a different slang expression might be the direct source of the expression as used there. Consider these three examples from the 1890s. From "Good or Bad Water?" in the Sacramento [California] Daily Record-Union (August 19, 1893):
Q.—So there is no taxation in all this now?
A.—Not a cent.
Q.—And no increase of water rates?
A.—Not unless you make them yourself. Fish ["the Worthington pump man"] can't do it.
Q.—But where does he get off?
A.—Why, in the sale of water to a growing city for twenty years.
From "Boon to Honolulu: B. F. Dillingham Organizes New Plantation Company," in the [Honolulu, Hawaii] Hawaiian Gazette (December 15, 1896):
"If Hackfeld & Co. are to be the agents for the company, Mr. Dillingham, where do you get off?"
"My interests are largely in the 0. R. & L. Co.; what benefits it helps me. By the consummation of this deal the railway company disposes of a lease of 3,300 acres of land, and Mark Robinson, who has large land interests here, will furnish the land on which the pumping plant and buildings will be located. ..."
And from an untitled item in the Chico [California] Record (September 14, 1897):
The [coal] producers have got their protection to the extent of 30 cents a ton on coal and they have raised the price correspondingly to the consumer. But the man by whose labor it is produced—where does he get off? He asked simply for 15 cents more per ton and he got—cold lead. Is this not an unanswerable showing of the folly and inconsistancy of taxing the consumer to add to the income of the coal trust with the expectation that it will benefit the laborer.
In each of these instances "where do you get off?" amounts to asking "how do you profit from this arrangement?" A fourth instance from the 1890s could be read this way, as well. From "Have Shut Up Shop: Silver Republicans Have Closed Sate Headquarters," in the Salt Lake [City, Utah] Herald (August 13, 1898):
But what of the People's party which decided to fuse with the silver Republicans? Where do they get off at? This question is such a pertinent and grave one that time should be given for thought and the answer will doubtless not be formulated in time for publication before Sunday.
Here, the sense of "where do they get off at?" may be something like "where does that leave them?" or it may be "how do they turn the situation to their benefit?" or it may even be "how do they have the nerve to proceed on their current course?" Of the three options, the first seems to me to be the most likely and the third the least likely. In the "'Lulu' in Poker" story in the same newspaper eight months later, however, the only possible senses of the expression seem to be "how do you benefit from the present situation?" and "how do you have the effrontery to claim that you won?" It may be that the two Salt Lake Herald stories show a transition in the sense of "where do you get off?" from "how do you benefit?" to "how dare you?" At least in those instances, the transition doesn't seem terribly far-fetched.