http://protonmailrmez3lotccipshtkleegetolb73fuirgj7r4o4vfu7ozyd.onion/blog/updated-tor-site
In fact, the Tor Project (new window) , the organization that developed the Tor Browser and oversees the Tor network, helped us launch the first version of the Proton Mail onion site (websites on the Tor network are called “onion sites” because .onion is the top-level domain used on the Tor network) back in 2017.