Can't sign up to their services completely anonymous, if you try to sign up using Tor/VPN it requires SMS confirmation, a previous email (and they block temp addresses) or a donation. Suspicious. There's also some iffy stuff on how they encrypt your emails.

Also, in their old ToS (pre-2022 update):

we have access to the following email metadata: sender and recipient email addresses, the IP address incoming messages originated from, message subject, and message sent and received times. [...] We also have access to the following records of account activity: number of messages sent, amount of storage space used, total number of messages, last login time.

And where this data goes?

When a ProtonMail account is closed, data is immediately deleted from production servers. Active accounts will have data retained indefinitely. Deleted emails are also permanently deleted from production servers. Deleted data may be retained in our backups for up to 14 days.

Sure, this was an old ToS. New ToS:

We employ a local installation of self-developed analytics tools. Analytics are anonymized whenever possible and stored locally (and not on the cloud).

Still unremovable telemetry not totally anonymized. Now they aren't even clear on what those analytics are, aside from:

Our overriding policy is to collect as little user information (personal data included) as possible to ensure a private user experience when using the Services. We do not have the technical means to access the content of your encrypted emails, files, and calendar events.

Yeah, can assume they're collecting the same things as before.