http://protonmailrmez3lotccipshtkleegetolb73fuirgj7r4o4vfu7ozyd.onion/support/inactive-accounts
Simply logging in to any Proton service on our web, desktop, or mobile apps at least once a year is enough. If you are already logged in, you should use one of our products at least once a year (for example, access your inbox, read an email, connect to a VPN server, etc.).