However, I believe that apostrophes on plural acronyms are falling out of favour, much as the dots between the letters have.

Oh. I would have said that the apostrophe was once used only in specific circumstances (when its absence would cause ambiguity, i.e. when the s would otherwise cleave too closely to the abbreviation: we awarded five MScs this year) but nowadays is increasing in use. The old old way to do it was to repeat the last letter: pp for pages, mss for manuscripts.

In particular, POWs would expand to "Prisoners of War", so why is the "s" at the end, and why the apostrophe?

Would you rather write PsOW? As the its-vs-it's thing shows, clarity and lack of ambiguity beat consistency every time in this game.

Peter