They could, but they evidently don't (stop you from using the service). I have a phone with a very old version of Android such that newer versions of Signal can't be installed. So it carries this really antiquated version of Signal, and, sure! some of the newer features (groups, some of the image handling) don't work. Still works just fine for the core purpose (voice, texts) though. As far as I'm concerned, kudos to Signal for maintaining full backward compatibility as far as is reasonable.

Tangent: I’m not versed in Android (iPhone for me), but what’s stopping you from installing a newer version of Android (like LineageOS) yourself through rooting?

Usually, it's the fact that the device drivers are included in the image.

I do wish we had kept from the existing desktop OS ecosystem.

Apple's update schedule is a lot better in this regard for me (iPhone SE still gets iOS 14 FROM 2016)

Someone has to make that version first. Each phone model needs a specific one, I am guessing because of the way that drivers are handled?

Device not supported.