There should be a category: "Background Check" in the Android developer options. You can pretty finely tune or alter the automatically set priorities and permissions for background activity there.

I don't know exactly what the option is called but since Android 8 there is at least a toggle there per app. Later versions have lots more settings.

Not GP, but most of the apps I use that works without Google Play Services (specifically, FCM) have this problem too. Vendor-agnostic notification on Android, and as far as I know iOS, is still painful. Ofc Slack and WhatsApp works fine : they use the Google notification system.

I use signal a lot (probably has 30-60 minutes screen time per day), and still notifications are extremely finicky no matter what battery optimization settings I tweak. So I'm leaning towards some apps being blessed

It seems very plausible Signal deliberately curses itself somehow, given its extreme privacy focus.

I use twitch almost every day, its notifications are usually off by 10-15 minutes... /shrug

On Android, FCM notifications sent with priority HIGH are usually available within 30 seconds and bypasses Doze (unless you're on ultra battery saver mode). Notifications sent with normal priority run within doze windows, so every 15 minutes or more.

Unused apps also indeed go in different App Standby buckets, which while it shouldn't affect FCM notifications, it does on some older android versions (https://developer.android.com/topic/performance/power/power-...)

If you're not using FCM, well you're limited to having to check for notifications yourself, either within doze windows, or by registering a foreground service that keeps your app alive and checking.