Modish, youngish, girlish are accepted words. Is 1970-ish accepted too, or even understood to be an English word?
Acceptable or not depends on where you try to use it. New words are formed like this all the time, but in a more formal situation you would only use such words if they are already widely known.
The meaning of 1970-ish would depend on the situation. It can mean circa 1970:
They stopped manufacturing that model 1970-ish.
It can also mean in a 1970 style:
That lamp really looks 1970-ish.
I'm sure a few purists might object, but I would suggest that it will be accepted by the majority, in that they will be fairly clear in what is meant by the term.
Thinking about it further, I'm not beyond tagging -ish onto various other words or phrases myself. Hardly the heights of eloquence but it usually gets the message across, which is surely purpose of language.
Speaking in more general terms, when I append ish to a word, I'm usualy trying to convey the fact that I'm approximating.
So in your example, 1970-ish would mean around the period of the year 1970 and the listener would realise that I could be talking about any of the several years before or after 1970, but that I don't know which one in particular.
But it depends on the context in which you use it.
OED 2nd Ed supports -ish forming in your case.
2.2 Added to other ns., with the sense ‘Of or belonging to a person or thing, of the nature or character of’. These were not numerous in OE., whence only a few have come down to later times. Examples are folcisc popular, hǽðenisc heathenish, þéodisc national, inlęndisc inlandish, utlęndisc outlandish (which come close to the gentile group in 1); also męnnisc human, cildisc childish, cierlisc churlish. In later times this ending has become exceedingly common, sometimes in the earlier colourless sense as boyish, girlish, waggish, but chiefly in a derogatory sense, ‘Having the (bad or objectionable) qualities of’: as in apish, babyish, boarish, boorish, brutish, clownish, currish, devilish, doggish, doltish, dronish, foolish, foppish, goatish, ghoulish, hoggish, impish, knavish, mannish, monkish, mulish, owlish, prudish, roguish, selfish, shrewish, sluggish, sluttish, sottish, swinish, thievish, waspish, whorish, wolvish, womanish. (These have usually corresponding Ger. forms in -isch.) Also from names of things, with sense ‘of the nature of, tending to’, as in aguish, blockish, bookish, brinish, feverish, freakish, hellish, moorish; or from other parts of speech, as snappish, stand-offish, uppish. In recent colloquial and journalistic use, -ish has become the favourite ending for forming adjs. for the nonce (esp. of a slighting or depreciatory nature) on proper names of persons, places, or things, and even on phrases, e.g. Disraelitish, Heine-ish, Mark Twainish, Micawberish, Miss Martineauish, Queen Annish, Spectator-ish, Tupperish, West Endish; all-over-ish, at-homeish, devil-may-care-ish, how-d'ye-doish, jolly-good-fellowish, merry-go-roundish, out-of-townish, and the like.
In the sentence :
He is a 1970-ish.
I would understand his character is back to 1970s.