Apps were rejected if they had a link that would open a page that would give you the information that you have an option to get a cheaper price if you pay outside Apple. You can find on other comments (maybe on the other thread) the exact rule from Apple.

Yes of course, but that is surely still forbidden. The change is about communication outside the app.

I’m pretty sure they still can’t, since that happens in the app. The rule mentions communication outside the app.

And AFAIK even to this lame extent is only a narrow exemption for applications that sell access to content like article, movies, music, and books.

They cannot send you to a URL and still can't mention non-IAP payments from the app. Only in a bulk email to users. Emailing your own users and providing them a way to pay for your cross-platform service was disallowed before.

And it’s only because they’re so big that Apple hasn’t kicked them off the App Store.

Not really, apps that distribute content like that have are allowed to take payment outside the app, I’m pretty sure it applies to anyone. No doubt the exception was introduced because of Amazon and Netflix, but i think it applies to anyone.