Plurals of acronyms, letters, numbers — use an apostrophe or not?

The Chicago Manual of Style says:

Capital letters used as words, numerals used as nouns, and abbreviations usually form the plural by adding s. To aid comprehension, lowercase letters form the plural with an apostrophe and an s.

The lowercase letter exception presumably exists because omitting the apostrophe can make the sentence much harder to understand (for example, "mind your p's and q's" is much clearer than "mind your ps and qs"). I don't know if the rule was different in earlier editions of Chicago, but it's very much in keeping with the guide's philosophy of prioritizing clarity and simplicity over prescriptivist rules.

There doesn't seem to be any consensus on this at the moment -- different style guides have different opinions:

New York Times:

Use apostrophes for plurals of abbreviations that have capital letters and periods: M.D.’s, C.P.A.’s. Also use apostrophes for plurals formed from single letters: He received A’s and B’s on his report card. Mind your p’s and q’s.

But do not use apostrophes for plurals of abbreviations without periods, or for plurals formed from figures: TVs, PCs, DVDs; 1990s, 747s, size 7s.

The AP:

Use apostrophes to form the plural of single letters but not figures or multiple letters.

I think I agree with the U.S. Government Printing Office:

While an apostrophe is used to indicate possession and contractions, it is not generally necessary to use an apostrophe simply to show the plural form of most acronyms, initialisms, or abbreviations, except where clarity and sense demand such inclusion.

[emphasis added]

It doesn't matter how many different authorities/style guides are cited - usage in this area has never been fixed, so it doesn't mean much to suggest the "rule has changed over time".

The use of apostrophes has always been less common, but it's been around at least a century (there, for example, the 1700's). It's also worth noting that (particularly in earlier years) writers would sometimes simply leave a space (i.e. - write the 1700 s).

Per other answers, I suggest only using the apostrophe in contexts where not doing so would make things hard to read (as in dotting the i's and crossing the t's). That's in line with my understanding that the general trend over many decades has been a reduction in the use of all punctuation where the primary justification was to follow "rules of grammar". Increasingly, the primary (or only) justification for punctuation marks is improved legibility.

Hmm. I recall having learned (in the '80s, in the U.S.) that letters, numbers, and words used as "themselves" (I would call these literals today, partly due to being a programmer) do take the apostrophe when forming the plural. Examples that mostly corroborate this, taken from a dead-tree American Heritage Dictionary, 2nd College Ed. (c) 1982, 1985:

42's and 53's

x's, y's, and z's

in the 1700's

an article with too many also's

I actually don't think I learned years that way (the rationale for leaving off the apostrophe for years would be that the digits in years are not functioning as figures per se, plus I'm sure I learned that '70s should have the leading apostrophe to mark the omission of digits but not the pluralizing apostrophe).

So it appears you have good support for using the apostrophe for single letters (everything we've seen so far) and some support for numbers (the old dictionary I cited here). This source doesn't say anything about pluralizing abbreviations of any kind, though.

Finally, I will chime in with everyone who recommends being sparing with apostrophes. I would really love to not feed the common tendency to pluralize randomly or overzealously with apostrophes. The less people see I bought 3 CD's, the less people will be inclined to write I bought 3 apple's. (I hope.)

The question is has the rule changed?

Yes. For example, the Webster’s New Collegiate Dictionary, 1981 edition, gives this rule in the section “Handbook of Style”:

Apostrophe ’

• often forms plurals of letters, figures, and words referred to as words
  ⟨You should dot your i’s and cross your t’s.⟩
  ⟨His l’s and his 7’s looked alike.⟩
  ⟨She has trouble pronouncing her the’s.⟩

No examples or guidance are given for omitting the apostrophe. This is different from the common advice today to use omit the apostrophe except when unavoidable.

My copy of Practical English Usage, 2nd Edition (Michael Swan, 1995) says this:

Apostrophes are used in the plurals of letters, and often of numbers and abbreviations.

• He writes b's instead of d's.
• It was in the early 1960's. (OR ... 1960s.)
• I know two MP's personally. (OR ... MPs.)

Using Amazon's Search Inside The Book I can see that these recommendations haven't changed in the 3rd edition, published in 2005.

So there's at least one source that agrees with what you were taught!

See Lynne Truss's 'Eats, Shoots and Leaves' (2003). She writes on p46:

only one significant task has been lifted from the apostrophe's workload in recent years: it no longer has to appear in the plurals of abbreviations ... . Until quite recently, it was customary to write "MP's" and "1980's" - and in fact this convention still applies in America.