They do. I use the protonvpn flatpak on some distos.

Cause nobody cared then and nobody cares now.

Credit Card only it looks like. Mozilla VPN is just a wrapper around Mullvad (it uses their servers and technology) so I would recommend getting that. Mullvad accepts a lot of more bespoke payment methods (crypto, wire transfer, cash in an envelope). The main reason to get Mozilla VPN is to bundle it with Mozilla's other services.

no, it also accepts cash and bitcoin

I think it's the same deal as search engines : people have been made too unaware of what is on their devices. Every service needs its own app or people will just not dare to "tinker".

A thought, as I don't know the answer, but a lot of these VPN providers provide access to VPN exit nodes in multiple countries. So the VPN app could provide an interface to choose the exit node. And of cause other functionality like reminding you that you need to pay.

They usually offer functionality thats not built in such as kill switches and split tunneling iirc

I think it's rebranded Mullvad

Why, because you can get the normies VPNs for 1 dollar per month or so? How do you think they make up for those low prices? Take a wild guess.