Is it definitive that these issues persist on Linux as well, or does that need to be tested again?
It seems to me like all providers (IVPN and Mullvad) require some manual steps on Linux to set up a killswitch (see: ProtonVPN IP Leakage on Linux and Workaround - #19 by certainty).
If that’s the case, we could just add instructions to Proton’s section on how to configure something similar, even if Proton doesn’t provide a guide to do so themselves like IVPN and Mullvad do.
I am leaning towards a criteria change because I think reliance on a killswitch feature for anything more than convenience is a mistake no matter how it is implemented. Similarly to Tor Browser, we should recognize that the most robust way to completely isolate this traffic is always a VM-based solution, in Tor’s case with Whonix (in Qubes). Similarly, as the PrivSec article at ProtonVPN IP Leakage on Linux and Workaround notes, Proton’s issues are also resolved with a strong Qubes setup.
I think that anyone who will be majorly impacted by this problem should be using Qubes, and killswitch implementation quirks should be considered acceptable by non-Qubes users, although we should indeed note what they are just for their reference, but not as a “problem” with the service.