Wouldn’t the OS vendor just block the installation of certain apps? Say, UK says that Samsung and Apple should block certain encrypted messaging apps. Regardless if it’s from App Store or not.
What you're suggesting would be a escalation and we should worry about that when it happens. Package names (the real name of an app on Android) can be randomised, for example.
The problem right now is that if you own an Apple device in Brazil, you can't install the app of this VPN provider. That doesn't sit right with me. Android has its issues, but if I can find the apk, then I can install it. What can you do with an iPhone? Perhaps use a 3rd party Wireguard or OpenVPN client that hasn't been blocked yet and download a profile from Proton's website? What happens when that client is blocked too?