Media Session was supposed to be the example session manager. Over time it developed more and more features but it's still supposed to be more of an example/reference than the one actually used. Having it in the pipewire tree gives it more prominence than is called for, so we decided to move it out of pipewire into its own project repository.
It is now available at https://gitlab.freedesktop.org/pipewire/media-session
The integration is now similar to WirePlumber - a subproject that is built if selected.
Note that this also removes the pw-reserve tool that relied on one source file of media-session, that tool is now built as part of media session.
Still missing: the docs integration, the media session docs are built but not published anywhere.