MarsSeed [1] filed a deletion request for qt5-multimedia-plugin-ffmpeg
[2]:

Not needed by any packages.
One-person project started and discontinued in 2021. [a]

- In total it has 20 commits. [a]
- 0 issues ever, 0 PR's.
- no documentation.
- only Qt5
- developed upon ffmpeg 4.1 and ffmpeg 4.4 only
- no automated building and testing whatsoever
- zero test code
- readme hints that it was developed on Windows only
- weird architecture: supports QtAudioOuput for audioplayback backend,
but prefers SDL2 (why?)

Arch repo's qt5-multimedia relies on gstreamer, but through gst-libav
optional plugin, it can use ffmpeg.

Packaging broken but no one complained:
- cannot be built because of missing makedepend 'qt5-tools'
- source link is changeable file (GitHub 'main' branch zip file)
- no source checksum checking

Package has no comments or votes.
Seems it is not useful to any user.

[1] https://aur.archlinux.org/account/MarsSeed/
[2] https://aur.archlinux.org/pkgbase/qt5-multimedia-plugin-ffmpeg/