There’s been a couple of threads** recently where people on the Roon team have suggested contributing to Musicbrainz, but I agree that this may not be solidified as official Roon policy, and I should not be giving that impression (not being an employee of Roon, I certainly cannot speak for them). And I also recognise that members of the Roon team have made it clear that Musicbrainz is not the perfect panacea.
I agree that there’s “no official guide” - partly because this is all a work in progress, and whilst I might regret the omission, it is what it is, and I can understand that.
I still can’t get my head around using the composer’s name in the Track Artist field though. Unless the composer in question is the actual performer of the track, and then it is valid. There is a tag for the Composer - why isn’t that used?
** I need to search for them, and give links; watch this space. OK - not so recent: How about getting albums added to the meta data source?