Apple has not spelled out clearly that this is okay for them to do, which means that it is a risk.
The user would be receiving a literal iOS notification from this app that talks about upgrades available outside the app.
Requiring no app generated notification has a clear line.
Requiring no email, which is a general mean of communication, has too big an implication. What about sending a text? Or a mail? Or to a secondary email?
I’m not a lawyer, but such a clause in contract would be deemed rather unjust, wouldn’t it?
It feels like a lawsuit waiting to happen.
Lawsuits against Apple's App Store policy enforcement are already in progress, but Apple has the best lawyers money can buy, and rulings are years away.
If a substantial part of your business risked being terminated at the click of a button by some unelected person that you have never met, with no proper way to appeal such an unjust decision, you'd probably think twice about doing something that could be misconstrued as intentionally violating App Store Guidelines too.
I blame Apple for the lack of clarity both in their rules and their enforcement. I don't blame ProtonMail for trying to adhere to Apple's arbitrary rules that often make little sense.
Apple wasn't crazy enough to go after Amazon Kindle app for asking the user to buy ebooks on Amazon.com, which bypassed Apple's system.
Probably---hopefully---they aren't crazy enough to fight a email client for sending emails regarding upgrade plans.
They already supposedly went after apps that linked to a privacy policy on their main website because the main website itself had premium options for sale. If you can't link to legal policy etc. because it might be a level or two of indirection from offering a premium service for sale through the site you have to do something like a special site just for Apple users. I can totally see them going after email and arguing Proton mail could hide those notification emails from iOS users if they contain links to premium.
However, I don't see how that can be solved. An email provider has every right to send an email to a customer informing them about anything, including a trial expiration and if that email contains a link, it is handled like a link from any other email in the app. It would be really silly for Apple to ask the provider to censor their user's inbox and hide a particular email they received.
I agree completely. If Apple agrees, they should spell this out clearly in their terms.
As it is, Apple gets to selectively enforce this whenever they want, which is unfair to both app developers and users.
One likely danger of spelling this out from Apple's point of view is that someone might say the same thing about other messaging services besides email. Take Facebook Messenger, for example. Imagine that Facebook started offering a paid upgrade to Messenger. Why should Facebook Messenger care on which platform you're receiving an internally-generated call to action to upgrade to some hypothetical paid plan? It's not like they were intentionally targeting Apple users in this hypothetical scenario, and it should be their right to not have to censor messages they decide to send to their users just based on the platform being used. It's (hypothetically) just a regular Facebook Messenger message, after all, generated by some marketing system completely disconnected from both the app and the messaging platform!
One thing that confounds the issue for ProtonMail specifically is that the app only works with ProtonMail, and ProtonMail is operating both the frontend and the backend. It's not like the Gmail app that also lets you use other email accounts with it as a general purpose email client. This is why I chose Facebook Messenger as an example -- it is a messaging platform, but both the frontend and backend are operated by Facebook.