I have to admit that I'm not all that knowledgeable about licensing, so I did a little research and you are totally right. My bad! Legal means would totally be possible then, but that'd require someone actually pressing charges. Since that hasn't happened yet, I fear it's pretty unlikely at least in the foreseeable future..
znapop
I'm currently running ArrowOS which is similar to LineageOS by being a minimally feature-enhanced fork of AOSP. I get most my Apps from F-Droid and the few proprietary Apps I can't do without I get from Aurora Store. Instead of installing GApps I use microG - a "free software clone of Google’s proprietary core libraries and applications" - for the few apps that need its notification or location services.
Now before anyone gets all riled up, I know this is far from ideal, but I've found it to be the best compromise between honoring my privacy ideals and providing a usable device that doesn't leave me tearing my hair out. YMMV and that's perfectly fine.
[..] legal ground to force proprietary module vendors to release their source code?
What would make anyone think that? No one has to publish their code. If you really want a FLOSS kernel you'll have to find open replacements for those modules or make them yourself.
Gorhill archives all related repos, so I'm pretty sure active development has enden.
The only "proper" fork I know about is nuTensor:
https://github.com/geekprojects/nuTensor
It has just been forked to provide maintenance though, there probably won't be any new stuff.