Only a generation ago, it was widely taught to include the apostrophe. Even then it wasn't universal, but it was one recommended way to do it.
For example, I have on my bookshelf "Webster's NewWorld Dictionary, Third College Edition" (copyright 1994) which says on p. 1560: "An apostrophe is used ... to show certain coined plurals. ... Figures may take either an apostrophe and an s or an s alone. 1990s or 1990's".
It also mentions a rule about using apostrophes with plurals of abbreviations if the abbreviations include internal periods, and it gives the examples of "Ph.D.'s" and "ICBMs". I guess internal periods have fallen out of favor too.
Anyway, the overall point here is it was taught this way relatively recently, and I don't think language moves so fast that what was taught a generation or two ago isn't considered acceptable usage. I'd call it correct but archaic.
https://english.stackexchange.com/questions/13631/is-an-apos...
Almost all the style guides recommend no apostophes on the end. Rather, apostrophes are reserved for numerals that are lefts out, i.e., '60s or 1960s, but not 1960's.