Are ffmpeg56 and ffmpeg54 packages discontinued?

libavcodec-ffmpeg-extra56 (FFmpeg library with additional de/encoders for audio/video codecs), libavformat-ffmpeg56 (FFmpeg library with (de)muxers for multimedia containers - runtime files) and libavutil-ffmpeg54 (FFmpeg library with functions for simplifying programming - runtime files) have been upgraded in Ubuntu 16.10, 17.04, 17.10, 18.04 and 18.10 to libavcodec-extra57 libavformat57 and libavutil55.

The same packages have been upgraded in Ubuntu 18.10, 19.04, 19.10, 20.04, 20.10, 21.10 and 22.04 to libavcodec-extra58 libavformat58 and libavutil56.

Open the terminal and type:

sudo apt-get install libavcodec-extra57 libavformat57 libavutil55

Usually when a package is updated its name does not change. In cases like this, the name changes because there has a binary-incompatible update to the library, which means that software compiled to work with the older version would not work with the new version. This means that you can't use the newer ones as a drop-in replacement for the older ones, since the software would need to be recompiled (and patched, usually).

The package name change allows your system to have both versions installed side by side in case some applications depend on the older version and some depend on the newer, and it means that the "newer" packages won't be marked as upgrades or replacements to the "older" ones, because they are considered separate and used by separate software.

In this case those particular older packages are no longer present in yakkety, meaning that no packages in yakkety depend on them anymore and they can be removed to clear up space. The likely explanation is that the software in yakkety has been compiled against the newer versions.

You don't use these packages directly so you can safely just depend upon the dependency handling of APT to ensure the correct ones are installed - if APT lets you remove the old ones (or aptitude search ~o shows them as obsolete) then you can remove them.