Apart from SimpleLogin, service that they acquired - all their service APIs are down. This to me indicates, that proton managed to create a single point of failure for all their in-house services, but didn't manage to put SimpleLogin under that umbrella yet.

If something can take almost everything down - Pass, VPN, Mail, Calendar - i.e. pretty much all your services that are seemingly standalone, then something's wrong with how you're managing these services.