It seems to be that these vendors ship builds of Android with aggressive battery-optimized defaults that breaks applications that rely on being able to run in the background.
Most push notifications are spam, and even when they aren't, they are usually from self-important apps that think they are the most important thing running on your phone.
The only push notifications I want are messages from humans.
See - messengers use push notifications to let you know about messages from humans. Most users of messengers get (understandably) upset if they don't receive notifications for incoming messages.
Note that you can always block notifications for apps you chose. The problem here is the other way around: You do want notifications, but even though you disable battery optimizations and some other things for the app you're using you can't get push notifications to be delivered reliably. That's a pain.
It seems to be that these vendors ship builds of Android with aggressive battery-optimized defaults that breaks applications that rely on being able to run in the background.