The Linux SDK download link on Develop for Playdate is
https://download.panic.com/playdate_sdk/Linux/PlaydateSDK-latest.tar.gz
which redirects to
https://download-cdn.panic.com/playdate_sdk/Linux/PlaydateSDK-2.5.0.tar.gz
which in turn responds with 404. Luckily the server has enumeration enabled so I was able to see that that file exists but has the wrong extension. It is at
https://download-cdn.panic.com/playdate_sdk/Linux/PlaydateSDK-2.5.0.gz
(but it's still a zipped tarball, so after downloading it should be renamed to .tar.gz).