I probably confused you with the Extended tags bit, I just mean for display purposes. I added the "display" word into my initial post.
What I mean is that after MusicBrainz tags got their special mixed case mappings in Mp3Tag v2.89, they are displayed in all caps and with underscores in Extended Tags, so I just wanted the 4 tags that I mentioned to be displayed the same way so it would be consistent for all MusicBrainz tags (yes, AcoustID is also a MusicBrainz tag, I asked a current Picard dev about that).
What I mean is this: https://i.imgur.com/KZ7LZkf.png, you can see the missing underscores not being displayed for the tags there. Also I'm not sure about whether they should be displayed as MUSICBRAINZ_ALBUM_RELEASE_COUNTRY or MUSICBRAINZ_ALBUMRELEASECOUNTRY, but I think Florian named the current tags so he can decide how they should be named. Anyway the Extended Tags display is not that big of a deal, I just like things to be consistent and couldn't not notice this
I think this is similar to what was already changed for other MusicBrainz defined tags like MusicBrainz Artist Id. These are specifically defined tags with a specific meaning, and their casing should be preserved. MP3Tag already contains a fixed mapping of most of those tags, but not for the ones @culinko mentioned above. See also this older discussion: