Radicale has a lot of different setups like stand-alone or via apache, with/without wsgi etc. The documentation is not very aware yet of all the pitfalls you may step in.
In at least one of these caess, apache / wsgi / radicale, it's unclear how to make thunderbird (lightning) work with radicale 3.x as for the 2.1.x series.
See here: https://github.com/Kozea/Radicale/issues/1142
Treat this post just as a reminder for others to make this critical 3.x issue visible.
BTW: I found it way to early to remove all of the 2.x ebuilds entirely. I had to recreate my own 2.x ebuild overlay to downgrade. This is a workaround until 3.x solves the issue.
thanks for pointing me about this issue. I have opened a PR: https://github.com/gentoo/gentoo/pull/20407 in order to restore the 2.1.12 ebuild and have set slot 2 and 3