If you communicate your expectation visibly, then this works. Otherwise not so much.

Even if they communicate visibly it doesn't always work. I don't use slack / pagerduty (not even sure what that is) and I'm not going to install or set up an account on some random proprietary service just to meet the demands of one email recipient. It might be fine in certain contexts (e.g. team members or friends/family who all use the same communication apps) but it breaks down when you're communicating with arbitrary members of the public.

Exactly. But it also works the other way. If you expect people to read your emails within 24 hours, make it very clear that that's a business need. Otherwise some of them will not.