(Making this a pinned thread until we have better instructions/links on the website.)
OK, ready to have some fun?
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Make sure you have a 64-bit capable Raspberry Pi and node v16 installed.
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Download the headless package from https://plexamp.com
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Extract in the pi home directory.
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Run from terminal to start (and ignore anything which is printed after you enter the code):
$ node js/index.js
Starting Plexamp.
Please visit https://plex.tv/claim and enter the claim token:
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You should now be able to hit http://<ip.of.pi>:32500 from a browser and get the Plexamp UI (you’ll need to sign in again in the browser, otherwise it would be a security risk). When you change player/cache settings, you’re changing the Pi settings. When you change UI settings, you’re changing app/browser settings. Play some music! Or control it from any other instance of Plexamp. (The browser portion was mostly for free and seemed cool and useful, but there are definitely things to fix up in there.)
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Install the Plexamp service if you feel like having it start on boot (and why wouldn’t you?). CTRL-C out of the foreground running Plexamp and do this:
$ sudo cp plexamp.service /lib/systemd/system/
$ sudo systemctl daemon-reload
$ sudo systemctl enable plexamp
$ sudo systemctl start plexamp
Congratulations, you’re now running the latest and most advanced version of Plexamp on your Pi! 
If you’re into Hi-Res audio, make sure to enable sample rate matching, and pretend you can hear the difference.
As I think I said before, I’ve only tested with a PiFi Digi+ board, which you can get super cheaply online, e.g. here. This provides optical/digital outputs, which is what I was looking for. I’ve additionally made some tweaks so that it’s the default audio output.
