They are not comparable, ffmpeg-python just abstracts away the CLI, pyav is a low level binding of the ffmpeg libs.

It may seem "dead" but ultimately it just helps you build CLI commands in a more sane way, the CLI interface to ffmpeg has been consistent for a long time. Only thing that may change is individual filters which you can just give raw to ffmpeg-python.

I remember when I was heavily using it last year I found a fork that seemingly had more sane typing or something but since LLMs last year didn't know about the newer lib but could write decent ffmpeg-python code I stuck with it and it did the job.