If a child left a note on the kitchen table to tell his mother about his report which is more clear:
Mom, I made all a's on my report card.
Mom, I made all as on my report card.
This proves for clarity, you need to use 's to form a plural in rare cases. I learned the expression is "I am learning my ABC's and 123's in kindergarten." Mind your p's and q's means mind your pints and quarts which came from the Gold Rush days in saloons where bartenders kept a chalkboard tabulation of the pints and quarts ordered by customers, so they could pay up later. They would say “Mind your p's and q's” so people would reference the board and control their spending on liquor.
The grammar rule for using 's when expressing 123's, ABC's, A's, B's, C's, 1920's, 1940's, 1960's, 1980's and so on has existed for centuries.
Now, it seems to depend on which style guide you are using. Words into Type, third edition (1974) takes this view:
In referring to decades, the sixties or the 1960's is generally preferred (not '60's, '60s, 60's, or 60s; the last form is used occasionally for ages of persons).
The New York Times Manual of Style and Usage (1999) agrees with Words into Type about the apostrophe, although about little else:
decades should usually be given in numerals: the 1990's; the mid-1970's; the 90's. But when a decade begins a sentence it must be spelled out; often that is reason enough to recast the sentence.
For centuries, 's has been used to indicate some plurals in rare cases.
Shakespeare wrote: “By my life this is my Ladies hand: these bee her very C’s, her V’s, and her T’s, and thus makes she her great P’s. It is in contempt of question her hand” (Twelfth Night, act 2, scene 5 [1st folio, 1623]; and note the absence of an apostrophe, and the plural ending, in the possessive “Ladies”).
In its first eleven editions, CMOS advised writing “the three R’s,” after which it became “the three Rs.” But the intent of the rule has remained the same: use an apostrophe wherever it is needed to prevent a misreading. And as anyone who got A’s in chemistry (or knows their Agatha Christie) might tell you, sometimes an apostrophe can spell the difference between a letter grade and a poison.”
Consider that many grammar school age children are still being taught the way people were taught in the 1800's. There are still new books instructing this way. Take note of the titles of these books for elementary grades:
“ABC's And 123's: Writing Practice Workbook” by author, D. C. Kail in paperback released on September 2, 2017, and it is available on Amazon.
“The ABC's and All Their Tricks: The Complete Reference Book of Phonics and Spelling” by author,
Margaret M. Bishop.
These exist for pre-schoolers: a book and a video from 1998 called “Blue's Clues ABC's and 123's.”
If this rule was good enough for Shakespeare, I don't see why the CMOS and other style guides have changed it. Many known style guides have NOT changed the centuries old rule.