Do not give them permission to your clipboard. It is possible today. I copy and paste passwords and I clear the clipboard afterwards, and I do not use junk like Spotify, and were I to use Spotify, it would be through the browser, not the application. Were it the application, it would be firejailed to oblivion.

It is possible to restrict clipboard access when running applications inside Firejail, i.e. Firejail allows you to restrict access to X11 and Wayland sockets, which prevents the sandboxed application from reading or writing to the system clipboard. See: "--x11=none", "--private=...", "--private-tmp", and so forth. You can run a GUI app with isolated clipboard via "firejail --x11=xvfb app".

For Wayland, you should block access to the Wayland socket by adding "--blacklist=/run/user/*/wayland-*".

I do not use autofill on desktop at all. I use it on Android, however.