Thanks for the data @mbgheban. I'm not completely convinced it's a hardware failure yet. For me, it definitely coincided with LineageOS installation--that doesn't mean the LOS installation caused it, but it is suggestive. If it is a hardware failure, I suppose I'm out of luck.
If it is software, I wonder: is there a way to install original camera software / firmware / drivers / crypto keys without building my own LineageOS?
I had exactly the same problem. With the installation of Los I started to have the first camera issues. Firstly, it was just bad quality, then green stripes, then drop outs, now nothing at all.
In the process, i had done a complete revert to stock just for clarity. Nothing changed. The camera was gone for good...
I did some reading and it appears the back cameras of the earlier days have issues and are to be exchanged in case of any problems, no questions asked.
However, it also occurred to me that the constant flashing of different (beta) ROMs had its role in this play. Maybe some bad driver was just giving the camera too much juice for a while?
Anyway, once i return from my travels, i will have to get a replacement camera. Good thing is, they're pretty cheap
and tutorials how to do it are easy enough to follow.