Slack gets used as Command and Control servers for malware and bot networks -- same as Discord. They are commonly installed so their network traffic isn't suspicious. They're also free. Blocking "hidden email" services is an easy first line of defense. You also can't register with mailinator.com email, for example.
People are constantly trying to use any service for illicit purposes. A somewhat-easy way to deal with that is to just ban them. Because an email address is used as username and an email address usually maps 1:1 to a person, this means you just got rid of a Bad Person.
This breaks if access to new email addresses is trivial. You now have to invest actual effort into validating that a new account isn't someone trying to do bad stuff.
Or you can just block all the services providing easy access to email addresses and outsource the issue to big providers like GMail and Apple. It's a lot less effort, and you'll lose near-zero legitimate customers...
True, but Apple's hidden emails are tied to a real credit card (not free) and can't be generated in bulk (AFAIK).
I'm not inclined to give any service the benefit of the doubt here until there is some evidence that Apple emails are indeed being used for this purpose. I haven't seen any such evidence (but I'm open to it).
Wow, that's pretty scummy behaviour! I wonder what the rationale is behind this?