There should be a kill switch function on the browser extension (in my case its Firefox). NordVPN and other competitors have this ability. It's the only thing preventing me from buying a full subscription.

It's especially important on MacOS where Proton themselves recommend to use browser extensions instead of the desktop app for split tunnelling due to Apple's webkit based apps messing with it. ( https://protonvpn.com/support/protonvpn-split-tunneling#macos )