You can sign people up. They can upgrade on your own site. As long as you don't tell them they can do that in the app.
Spotify don't. They were communicating the promotion via email, outside the app, where Apple has no jurisdiction.
A tonne of companies aren't paying Apple 30%. It's basically on a case by case basis -- if Apple likes you or not. They don't like Spotify.
At a bare minimum, apps must be allowed to explain to users in-app that they can reduce their payments by paying outside the app.
I've run the numbers on products like this before. In order to absorb transaction fees, build your own product, and support it, fees need to be in the 10-15% range.
Also, note that almost no companies charge less if you pay out of app -- the company just keeps more for themselves.
Apple does charge 10-15% for Spotify like apps viz. apps that sell subscriptions for more than 1 year.
https://developer.apple.com/app-store/subscriptions/whats-ne...
After a subscriber accumulates one year of paid service,
your revenue increases to 85% of the subscription price,
minus applicable taxes. All current subscriptions are
eligible.They don't like competition.
FTFY
At the company I work, we have an app for a customer where you can purchase things on their site. For a long while you could sign up in the app until one day where apple literally said (on a super minor bugfix upgrade): "If you are signing up users in your app, we are contributing to your revenue. We do not want to do that, unless you are using in app purchases".
This particular app could not use IAP for technical reasons, so we had to remove sign up from the app.
That is how the rule works
edit: Although, your point about the case by case basis could apply here, although this is not on a spotify scale, I assure you :)