So steam deck is arch based. I just installed arch on my desktop with gnome. I couldn’t get proton vpn to work at all until I installed network-manager-applet. Then it worked fine. (It wouldn’t connect). I had to do a ton of googling before I found that fix, I think a lot of people give up on proton vpn before they find that solution/dependency. It sounds like other packages can provide what proton-vpn is looking for, but I think network-manager-applet gets installed often enough as a dependency where it just works for some people, while for others getting proton to work is a chore. (I’m pretty sure it’s not installed on a fresh install with gnome - making getting proton to work on a bare bones install challenging). I dunno if that or another package is already there on the steam deck - but worth checking that line of troubleshooting out.

Now, if it’s not there - I don’t know the full dangers of installing that package on the steam deck, maybe someone smarter can comment on that. It should be noted that I installed network manager with pacman and proton vpn from the AUR. I don’t know how the flatpak would behave - but it didn’t work either without network manager installed.

Good luck!