Tor connection via ProtonMail Bridge
While ProtonMail Bridge is connected, there is a lot of information included in the connection by necessity. If this connection were to set up a Tor connection to pass through to the onion version of ProtonMail by default, it would make it more difficult to censor or identify users who need heightened security. If it worked reliably, it could be a way to enhance overall security by both encrypting communication and also hiding the IP address.
I don't totally understand this, but Proton's Tor support seems DOA. No real updates since 2017.