/usr/lib/x86_64-linux-gnu/libavfilter.so is in libavfilter-dev 7:2.8.6-1ubuntu2.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libavfilter-ffmpeg.so
/usr/lib/x86_64-linux-gnu/libavfilter-ffmpeg.so