ProtonVPN did an API bump in this version: Version 2.7.56.1 (2021-06-18) which left everyone with an Android version older than AOS 6 in the dust. So I went to the archives and grabbed the version just before that one. Ran it for the first time, configuration wizard had no issues but as soon as I tried to reach out to the server it refused to stand up a tunnel saying my version was too old. Not only did they leave permacomputing folks behind for sustaining their still-quite-functional devices, but they proactively sabotaged us from the server side.
AFAIK they made no excuses for the API bump. The usual excuse is “for security reasons”... yeah.. bullshit. Anyway, here’s the workaround:
The absolute latest openvpn app still supports AOS 5 (somewhat suggesting there is no compelling security reason to force AOS 5 users to throw away their devices). Or if you have AOS 4 you can take the openvpn version from 2 years ago. ProtonVPN distributes openvpn config profiles and the openVPN app can simply import those.
Also worth noting that F-Droid warns of anti-features on the ProtonVPN app but OpenVPN is free of anti-features. That said, I got an authentication error, but I doubt that’s related to this procedure.
update
ProtonVPN is possibly breaking EU law. If someone subscribed to service less than two years before the forced obsolescence, ProtonVPN is obligated to continue service as long as necessary to serve the consumer for 2 years.
What you seem to be implying is that the OS removed an interface that the app is using. That’s unlikely because ProtonVPN needs generally the same facilities and mechanisms as openVPN, which was not forced to bump their SDK. You can also see that the ProtonMail app (which is inherently very different than VPN service) also bumped their version to abandon AOS 5 users a year later. Yet other email clients (which generally need the same interfaces) were not forced to abandon people.
more precisely, the theory I'm trying to express is that Android 15 & 14 has banned loading apps that target much older versions. A decision made by Google, forcing the developer to comply or see their app not work. So in my opinion your grievance should be targeted towards Google. Personally, I'm pissed that most of my humble bundle android apps collection doesn't work any more because this.
This does not explain the fact that ProtonVPN’s server proactively refuses to serve users of their client version 2.6.91.0 (which is the last to target AOS 5). Google has nothing to do with that.
But also, if your theory is correct then openVPN fails to install on Android 15 & 14, correct?