Just joined looking for some answers myself, but since I have been playing around with my project for 2 days now, thought I'd throw this out there.
Streaming to icecast using webm (vp9) appears to be broken on the current version of obs. I installed obs v23.2.1 and it works. For me v23.2.1 crashes when I click "stop recording", but seems stable while actively recording.
Note, I do not believe the issue you are experiencing with the current version to be a problem with OBS. I couldn't get it to work with the latest version of ffmpeg either. I had to roll back to ffmpeg 4.1.4 to get it to stream to icecast in webm.
-Alan