According to https://trac.webkit.org/wiki/WebKitGTK/DependenciesPolicy we can now bump to GStreamer 1.16.2: https://packages.ubuntu.com/focal/gstreamer1.0-x https://packages.debian.org/bullseye/gstreamer1.0-x
Are you working on this besides the bug-report (which is appreciated)?
Pull request: https://github.com/WebKit/WebKit/pull/6087
I struggled with 'Tools/Scripts/git-webkit pull-request', that's why it took longer. Now it seems to have worked out.
Committed 256284@main (b2517e465d9f): <https://commits.webkit.org/256284@main> Reviewed commits have been landed. Closing PR #6087 and removing active labels.
<rdar://problem/101925414>