Volen Also, their Terms say they might block access to mailbox account if its accessed by VPN (except their own VPN) which is also a big issue for me as I am using Mullvad VPN: https://proton.me/support/compromised-account-temporarily-locked
Ahh. I see what you're talking about now. This isn't in their Terms of Service. It's their security mechanisms locking out an account due to suspicious activity.
The full statement is:
It is also possible for our automated systems to lock you out of your account in error. This can happen if you use another VPN service (not Proton VPN), Tor, or if you are on the same network as someone who has been blocked by Proton for violating our terms and conditions (for example, if you use your work or college WiFi).
They also state that you should be able to recover your account with your configured recovery contact information.
Before logging in to your account, you may be asked to enter a code sent to your recovery email address or phone number
None of this strikes me as particularly different from other major email providers and their protections against account compromise. It does indicate some level of tracking "normal" behavior or traffic for an account. So, perhaps your threat model is against this and that's okay.
I think it's incorrect to claim that simple VPN usage is against their Terms of Service. A concern, yes, but not a clear violation.