Sigh. Yes, I know it is the internet. That's why I only kept it as simple as "does the word in question own something? If yes, use apostrophe, if no, don't use one"

Your statement:

"Abbreviations containing punctuation or mixed case letters: Ph. D's M.A.'s Mrs.'s"

is wrong. Regarding the lower case letters, your point is correct; however, my point is that people on the internet write about decades and centuries all of the time, so they might as well do it correctly. Not often do I have a chance to jump on someone about plural lower case letters.

Apostrophes and plural forms:

The general rule is that you should not use an apostrophe to form the plurals of nouns, abbreviations, or dates made up of numbers: just add -s (or -es, if the noun in question forms its plural with -es). For example:

euro

euros (e.g. The cost of the trip is 570 euros.)

pizza

pizzas

(e.g. Traditional Italian pizzas are thin and crisp.)

apple

apples

(e.g. She buys big bags of organic apples and carrots.)

MP

MPs

(e.g. Local MPs are divided on this issue.)

1990

1990s

(e.g. The situation was different in the 1990s.)

It's very important to remember this grammatical rule.

There are one or two cases in which it is acceptable to use an apostrophe to form a plural, purely for the sake of clarity: •you can use an apostrophe to show the plurals of single letters: I've dotted the i's and crossed the t's. Find all the p's in appear. •you can use an apostrophe to show the plurals of single numbers: Find all the number 7’s.

These are the only cases in which it is generally considered acceptable to use an apostrophe to form plurals: remember that an apostrophe should never be used to form the plural of ordinary nouns, names, abbreviations, or numerical dates.

Oxford dictionary

Fortunately, I do know what I'm talking about. Unfortunately, you failed to see the point and larger picture of concentrating on correcting the proper use of written decades on the internet.

edit: Forgot a ":" up at the tippy top.