> the format itself does not support truly glitchless playback, because it always stores an integer number of audio blocks, and adds padding at the beginning.
I agree, although arguably the MP3 "core format" has had a number of extensions around so many things that ultimately became de facto standards (vendor extensions if you will). I remember how you could not have precise seeking or current time display with VBR, which now seems to be now largely a solved problem.
> There are certainly nonstandard metadata hacks for MP3 to retrofit gapless support
One of these is LAME adding gapless stuff at least as far back as 2003, so yes it's a workaround to a format limitation, but it's quite an old one, and fairly stable and known well enough that it's been a while since encoders default to encode with gapless metadata.
There's an interesting historical explanation here:
https://sourceforge.net/p/lame/bugs/453/#1bd2
I do agree though that the easy path out of it is to just stop using MP3, for this reason and a zillion others ;)