I use divestos and I saw this coming when they failed to port to lineageos 21.0. I suspect they simply couldn't muster the effort (or funding) to continue.
I just want LineageOS with microG and relocked bootloader :-(
Relocking the bootloader is a bad idea unless you know it doesn't verify integrity on boot or have some way of updating the keys used (AFAIK, only Pixels properly implement that)
Relocking the bootloader isn’t really necessary unless you care about the evil maid attack. I’m slightly worried about it because I travel a lot and want to be sure that my phone stays intact in the unlikely case it’s seized at the border or something. (Of course, the data is encrypted, and you could reflash the OS when you get the phone back, so relocking isn’t strictly necessary but still nice to have IMO.)
I do have a Pixel; I think later OnePlus models also implement that but yeah, the support is really scarce.