Documentation bug: the front page at <http://id3lib.sourceforge.net/> stresses the close integration between this project and the ID3v2 standardisation effort (which I take to be <http://id3.org/>). Given that that page discusses a 2.4.0 version of the ID3v2 standard (cf <http://id3.org/id3v2.4.0-structure?action=raw>), it would be useful if the id3lib front page mentioned whether it supports this.
Looking at the output of bin/id3info from v3.8.3, I'm guessing that you don't support it yet; but I saw a passing mention of 2.4 in one of the bug reports, which seemed to suggest you did support it. Either way, it would be useful to mention 2.4 one way or another
Yes, it appears that v2.4 hasn't had much takeup (the 2.4 document is dated 2000), but Audacity at least appears to be producing v2.4 tags so there are at least some 2.4 tags out there in the wild.