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Lots of iOS apps have the option, but ignore it and send you push ads anyway. Apple may require it to be present during app review, but they don't seem to enforce that it's used correctly.

Does Google actively police app's use of channels? Is there any mechanism to stop apps abusing "time critical" channels and sending unwanted marketing?

They technically allow you to, but make it really annoying to. Uber for example:

Account > Settings > Accessibility > Communication Settings

Most apps are cross-platform. If you're already required to do it on Android, going out of your way to avoid it on iOS doesn't make a lot of sense.

It absolutely makes sense (in a capitalist sense). Then you get more money/engagement/whatever on all of the other platforms.

It's the same reason Microsoft built functionality to let users in Europe have links open in their default browser instead of Edge but blocked that feature for the rest of the world.