Exactly. Their VPN app on Linux does not even have WireGuard options, let alone the Stealth Protocol. No port-forwarding either. There are so mamy features on Windows that they do not provide Linux users.
I change servers very frequently. For that I have to download a lot of config files. Considering, I am paying Proton just as much a Windows user, I shouldn't have to do all this work.
I have a seedbox built with ProtonVPN and Linux. You can't use their app but the manual configuration does work. (Port forwarding is annoying though, i had to write a script to manage it.)
It will probably get made eventually, but they have said there is no ETA for now. In the meantime, you can upvote the suggestion on their feedback forums.
On another post about this someone said the closed beta for Mac is next then after the Mac version is done they will start on a Linux version. So it will happen just not now.