It's because gtk+ 4 will no longer support general-purpose loadable modules – the very feature which makes the palette possible. Also the maintainer is disappointed with the attitude of GNOME to third party developers: https://github.com/p-e-w/argos/issues/75#issuecomment-475844...
Man this is terrible! I was warmed to Gnome 3 (I like the experience unlike most of the loud crowd) but this is inexcusable. Literally a breaking experience for hundreds of third part applications for some reason I just don't understand.