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.
Because otherwise if you have too many small letters in a row it stops looking like a plural and more like a misspelled word. Because capitalization differences you can make more sense of As but not so much as.
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.
Why use for lowercase?
Because otherwise if you have too many small letters in a row it stops looking like a plural and more like a misspelled word. Because capitalization differences you can make more sense of As but not so much as.
That looks like an oddly capitalised “as”
That really gives the reason it’s acceptable to use apostrophes when pluralising that sort of case