What are you talking about? Why would it have been remastered since 91 repeated times?
It hasn't been, at least not by U2 in the manner they did the recent remasters. I'll say it again...Boy, War, October, TUF and JT have all been "remastered" in the past more than AB ever would be, and that didn't stop U2 from issuing genuine remasters of those albums.
Every time there's a format change (i.e. LP to CD, LP to cassette, cassette to CD, CD to vinyl) there's usually a remaster. The early U2 albums were "remastered" by Island in the late 80's/early 90's (I have an old copy of War I think that has a sticker saying
"The Island Remaster Series" or something like that). They apparently got another "remaster" in the late 90's (and perhaps AB was part of this, that I don't know). None of this was really publicized that much because it was just common practice before "remaster" went from being a technical to (also) a marketing term.
In addition, as you mentioned, all the tracks on the compilation albums were remastered for loudness (an unfortunate practice which is common on most new release and reissues these days).
Then there are foreign labels, who often make their own master...there's a noteworthy Japanese pressing of Joshua Tree with a supposedly good remaster, though I haven't heard it.
Record clubs (when they existed) often made their own masters, which most people assumed were inferior, though that wasn't always the case.
Then of course there's the excellent (and hard to find, and expensive) Mobile Fidelity (MFSL) remasters of War, TUF and JT, which were the gold standard until the new reissues came along (and some people still prefer them). In fact the MSFL remaster of JT was the first CD edition of that album to correct the One Tree Hill/
"Oh Great Ocean" fade out mistake that was present on the early Island release.
With noteworthy exceptions, changes are usually subtle on a remaster and you know have to know what you're looking for to hear the difference. Though in some cases the difference can be dramatic (but usually this would involve remixing as well as remastering).
A true remaster goes back the the original source tape, whether it's digital or analogue. The U2 reissues so far have been genuine remasters, supervised by The Edge. The clean up studio noises, imperfections in the recording, etc. Apparently U2 didn't do this for AB, for whatever reason. However like many I assume it will be at least "remastered" for loudness.