For example, the contents of the link. Media Control interfaces on Windows & Mac - Album on Imgur
Currently Plexamp only sends notifications, but does not send media controls to Windows SMTC
By enabling Plexamp to support SMTC, it will be possible to use such third-party services as Discord rich presence.
Plexamp scrobbling · Issue #94 · ungive/discord-music-presence
While there are other ways to provide last.fm rich presence, this would be another reasonable step to integrate into the windows rich/native framework, and surpass other media players.
It wouldn’t even surpass other players. It would simply bring Plexamp to some degree of parity with many other music apps, as well as most popular browsers, and the full-fledged desktop Plex clients.
Here’s a handy-dandy chart of what supports GSMTC: ModernFlyouts/docs/GSMTC-Support-And-Popular-Apps.md at main · ModernFlyouts-Community/ModernFlyouts · GitHub
You’ll note that Plexamp, Plex, and Plex HTPC are all missing, though Plex and Plex HTPC do indeed support SMTC. 
I ended up here while looking for how to bridge Plexamp to SMTC somehow, myself. But I don’t see any way to read any info from Plexamp. Plex really doesn’t document their clients well, especially considering how minimally they implement system facilities in Plexamp.
However, seeing as how Plex and Plex HTPC support SMTC, it shouldn’t be too much of a lift for Plexamp to support it.
As for OS compatibility, WSMTC was introduced originally in Windows 8.1 (but there’s some kind of compatibility issue between SMTC on 8.1 and 10, which has purged itself from my brain some time ago). Plex currently only supports Windows 10 & 11. So there should be no concerns around end-user compatibility.
Also, for testing SMTC I personally like to use GitHub - DubyaDude/WindowsMediaController: Allows developers to more easily get information from and interact with the Windows 10/11 OS media interface. (Also referred to Windows System Media Transport Controls (SMTC)) with its CLI and UI apps.
If your goal might simply be integrating Plex music playing with Discord, this is what I use for that today, but it simply just reads from the API and tries to do some stuff before writing to the Discord API. I keep it running on a remote desktop, so it will report across all my players. So, not really a solution for SMTC for “what’s playing on this machine?” alas. But perhaps a partial workaround, in the meantime, if you (like me) prefer Plexamp most of the time.
I would like to note that this lack of SMTC is really frustrating, considering it sounds like they want to remove Music support from the regular Plex clients. While the full client’s music player lacks many of Plexamp’s features, it also has features Plexamp lacks (such as SMTC) while also providing an interface which works better for “I just want to listen to this whole album and nothing else.”
This needs to be added, SMTC is supported by plex.exe itself (the main app from the server). Allready made another thread for it to be added. Would like to see Pano Scrobbler working with plexamp on windows and or linux, it would also solve the now playing issue/absence with last.fm.
