How to disable Peek / Heads Up notifications globally in Android Oreo? There are multiple ways to disable Peek/Heads Up notifications in previous versions of Android (e.g. these or these), but it seems that the changes introduced by Oreo (Android 8.X) prevents them all. Is it possible to disable Heads Up notifications globally, for all apps, in Oreo?

I mean without disabling all normal notifications, or other features, but just this one. Ideally with no root required, but root solutions are also ok.
It's possible to disable the heads-up notifications with adb, without root, using one of the two options below. Sadly, on some of the most recent LG or Samsung phones, this setting is saved only in RAM, hence it only works until the next reboot.
Preliminary steps (required for both options)
On your phone:

Disable &quot;Disable alerts during game&quot; feature (see this issue for details)
Under Android Settings/Developer Options, enable USB Debugging;
Under Android Settings/Developer Options/Select USB Configuration, enable PTP (Picture Transfer Protocol);
Plug your phone to your computer.

On your computer:

Install ADB standalone for Windows;
Install the drivers;
Open a command prompt to the folder C:\Program Files (x86)\Minimal ADB and Fastboot
Run ./adb devices to make sure your device is properly recognized;

On your phone:

press OK on Allow USB Debugging?

Option 1
On your computer:

Run ./adb shell settings put global heads_up_notifications_enabled 0

Option 2
On your phone:

Install SystemUI Tuner
Follow the instructions and run the required adb commands
Edit Quick Settings:



Add the Heads-up Notifications Toggle to quick settings:



Tap on the toggle to disable the heads-up notifications:


As mentioned above, both options work only until the next reboot on some phones which save this settings in RAM, so option 2 is more convenient because it allows to easily re-disable the heads-up notifications after reboot using the quick settings toggle.
Android 9 and above
It is possible to disable them from:

Settings &gt; Custom restrictions &gt; Don't pop notifications on screen (source)

If that still doesn't work try disabling and enabling:

Apps &amp; notifications &gt; Special app access &gt; Adaptive Notifications (source)