Am I misunderstanding? Isn’t this already possible on iOS? You can either disable notifications for individual apps or just turn on Do Not Disturb mode and leave it on. Fairly sure there is a setting to allow phone calls.
On iOS, apps needs to ask permission before they send notifications. If you missed that, there's also a single toggle in Settings -> Notifications for each app. There's no situation where you should have to "kill all the various types of notifications". While I agree a global "Disable Notifications" toggle would be nice, I don't think it's too much work to just tap "Don't Allow" when an app initially prompts you?
The iOS notification ecosystem is actually miles better than it was in the iOS 4-10 days, I think it's fairly user respectful now.
There are also trial notifications, which only show up in notification center. Every app can just do that without any permission.
https://developer.apple.com/documentation/usernotifications/...
Every app that I install, I have to go through the same routine of finding and killing all the various types of notifications it tries to send me. But I don't want apps to notify me of things. Ever. I'll check my email a few times a day, and the various messenger apps once every few weeks, and they will have a list of new messages.
I'm fine with the phone ringing when one of my contacts calls. But that's the full extent of where I'm prepared to allow it to distract me.