According to http://lilypond.org/news.html 2.21.X is a development versions. >If you want to use the latest stable version of LilyPond, we recommend using the 2.20.0 version. Would be nice to have ebuild for 2.20.0
But currently in gentoo repo 2.21.1 marked as stable.
stabilization in gentoo is not directly linked to upstream stabilization.
Where can I look up stabilization rules in Gentoo Wiki?