Please read the thread. To summarise: ProtonVPN doesn’t meet minimum criteria laid out by PG, as its macOS client has a broken killswitch.
Not the Linux client.
As far as I know, for the official WireGuard client on Linux, the “killswitch” has to be set separately. It isn’t built-in.
Not removing it or not changing the minimum criteria for VPNs right away then makes PG a “dangerous” source of information. I mean, we won’t tolerate this from other projects or orgs, so why Proton?