Had obs-v4l2sink working perfectly up until I upgraded Ubuntu to 20.10. Same kernel (I was on 5.9 before upgrading), same OBS, same everything. I've done a complete purge of OBS and reinstalled both it and v4l2sink to make sure that nothing got messed up in the upgrade, but that doesn't appear to be the issue either.

Symptoms are that there is no longer a v4l2sink option under the Tools menu, and there is no output from OBS to my loopback device anymore.