That’s half-right. Upper-case letters aren’t pluralised with apostrophes but lower-case letters are. (So the plural of ‘R’ is ‘Rs’ but the plural of ‘r’ is ‘r’s’.) With numbers (written as ‘123’) it’s optional - IIRC, it’s more popular in Britain to pluralise with apostrophes and more popular in America to pluralise without. (And of course numbers written as words are never pluralised with apostrophes.) Acronyms are indeed not pluralised with apostrophes if they’re written in all caps. I’m not sure what you mean by decades.