Make that pretty much all Linux Distros. Just write Linux, to be honest. Then have additional setup tabs for specific distros.

If it run on one Linux distro, it will likely simply run on other distros as well. There's no need to over-complicate this. Distro-specific packages are nice, but that is often done by voluntairs and distro maintainers.

I think they should start by offering packages for Fedora and Arch, as well as a Flatpak package. Perhabs a .deb package for Debian and Ubuntu derivatives. But those distros aren't really to be recommended as they are constantly outdated due to the update structure, their package manager isn't as intuitive to use and Ubuntu has done countless questionable things. Also, the KDE Plasma desktop environment is more stable, more intuitive for most people (beginners and advanced users alike), beautiful and stable. It's feature-rich and adopt universal protocols instead of removing things like the system tray and breaking apps like Gnome does from time to time (then refuses to work with others because they're too stubborn and narrow-minded).

Anyway, don't over-compliate this. Linux is Linux. Distro-specific packages is as simple as package configuration.

Arch users are paying customers too. Is proton saying that we need to change providers if we aren't willing to change distros?

Needless to say this is strongly supported by any Arch Linux user that's also using the Protonmail Bridge app. Crazy to think that just because Arch Linux isn't supported, we have to put up with Qt's version not being correct, and therefore having to run the bridge in cli mode. I expected better of Proton's engineers.