That’s really basic and not even the minimum for actual long-term support. Remember those ROMs needs to hack together pices of binary blobs and drivers scattered around stock ROM and do many patches. Basically any new Android version is doing the work once again, that LineageOS and other projects automated. And after the end of official updates they are stuck with untouchable firmware package.
What should be done is adding the support to the upstream Linux kernel itself. Like AMD and Intel are doing on desktops, thanks to that we can have almost lifetime updates, multiple choices of OSes and have one image for all devices instead of doing seperate builds. Or at least provide documentation and drivers so the community can do it.