As mentioned previously, priorisation between streams has been removed from the main HTTP/3 specification to be worked on separately.
This was due to learnings from the HTTP/2 prioritisation model and its implementation (or lack there of) in the real world.
A simpler prioritisation model than HTTP/2 has been proposed using HTTP header fields with a limited number of priority settings. This is a key change from the Dependency and Weight flags in the HTTP/2 Header frames and allows better understanding at the application layer.
Reprioritisation, and whether to support this, is still being discussed. HTTP/2 had Prioritisation frames to handle this but the true independent streams in QUIC and HTTP/3 makes this more complicated so the benefits versus the complexity are still being debated.
When (or if!) a better prioritisation model is agreed for HTTP/3 it is hoped to be able to backport this to HTTP/2 too, to address the complexity and implementation concerns there.