Nothing to do with a dark pattern.
That's not true. I'm talking about consumer rights. In fact, last I checked (about 1 or 2 months ago), Data Portability Right doesn't even exist in Swiss laws, but it does exist in GDPR and the California Privacy Act.
Protonmail is all built around the Swiss bank image of "your data is as secure than your money". Protonmail doesn't even offer IMAP for free.
If you agree with their methods, no one will stop you to use their services. But again : they do NOT comply with basic consumer rights common to other providers, and I wouldn't recommend them to anyone I know
Regarding the IMAP support, I don't see how it's an issue that they provide it to their paid tier. ProtonMail is a paid service, with an option to have limited free accounts.
Good luck when you have 3000 emails and you are physically disabled.
In other words, signing in and using the service is "free", but you must pay to quit it or move to another provider.
There are also many other data than just emails : contact list, your private keys, the folders/tags you used, and all other personal data that Protomail may have about you.
Facebook and Google allow you to download a single zip of _all_ your data, at any time you wish, _for free_. They do it because it is required by the _law_, not because they are being nice.
So, yep. I would send a big warning to anyone who wants to try Protonmail, because Protonmail's terms of service uses dark pattern in that regard, and probably illegal in EU or California.