Comment by pja

I believe the blocker here is that USBC display support requires getting full Thunderbolt support working. Apple built their own Thunderbolt chips, which diverge from the Intel hardware that everyone else has used & that the Linux kernel drivers are written against enough to make support much harder than you'd expect.

IIRC Asahi has finally acquired Thunderbolt support but that support still doesn't have the pieces needed for external displays.