How to disable fullscreen system location notifications?

Unfortunately, you currently cannot turn these off. It may be helpful to send feedback to Apple concerning this.

If you really want to stop them, you could go to Privacy > Location Services and disable location services on your devices. You could also go application by application on this list and change all apps that have the permission set to "Always" and change them to "While Using", but you likely have a reason to have them set to always to begin with.