About a month ago, a user posted to ProtonMail's subreddit complaining about how they recently had both of their 2 accounts banned. The reason was simply for having more than 1 free account.

Per their Terms and Conditions:

Having multiple free accounts is not considered an acceptable use of our service (e.g. bulk-signups, large number of free accounts created by a single organization or individual). Free accounts can also only be created and maintained by their effective users (e.g. it is not acceptable to create accounts in anyone else’s name and later transfer credentials to that third party).

The Company reserves the right to take actions against any free account considered abusive in that regard.

Assuming this was a 'CYA' term, and not something that they were actually looking to enforce, I asked their support directly. They confirmed that having more than 1 free ProtonMail account will get your accounts banned. This is also true regardless of how many paid accounts you have.

This isn't stated anywhere when creating an account, and in fact was not the policy until December 2020, when they quietly changed the Terms and Conditions, from the following:

Having multiple free accounts is an acceptable use of the Service, as long as it is done in a proportionate and reasonable way (e.g. bulk sign-ups or large numbers of accounts created by a single organization or individual are not considered reasonable use of the service). Free accounts can also only be created and maintained by their effective users (e.g. it is not acceptable to create accounts in anyone else’s name and later transfer credentials to that third party).

The Company reserves the right to take actions against any free account considered abusive in that regard.

Despite having two paid ProtonMail accounts (and several more, lightly used free accounts), I have had no choice but to seek alternatives.