From what I read they weren't setup to host multiple apps and needed a somewhat large refactor to do that. The calendar on beta.protonmail.com is probably what will be released to production.

The only thing to smooth over, from my understanding, is the fact that they did a lot of refactoring to become multi-app.

Disclaimer: My understanding could be wrong.

It is already available in Beta at beta.protonmail.com. We're looking to also put out the mobile apps around the end of this year or beginning of next year.

agreed on calendar. Overall I really enjoy protonmail