Policy takes effect April 9, 2025 for new accounts, or until April 9, 2026 for current accounts (a year or two). Just have to log in every year to keep an account active.

I think this is maybe part of a broader conversation that needs to happen about limited resources and data; like with Gab.com limiting uploading media to paid users. Sounds like protonmail probably has a lot of junk accounts they want to clear to keep their service running smoothly.

edit: https://proton.me/support/inactive-accounts