On the GrapheneOS website, the section "Sensors permission toggle" says the following:
When access is disabled, apps receive zeroed data when they check for sensor values and don't receive events. GrapheneOS creates an easy-to-disable notification when apps try to access sensors blocked by the permission being denied.
I was curious about this notification, so tried to deliberately trigger it, but I cannot see such a notification anywhere. I had imagined this would be similar to the "Play Integrity API" access notification that I see occasionally, but I'm not getting any notifications about a lack of access to these sensors.
I had initially tried disabling sensors in Pokémon GO, to see what would happen. There's a compass in-game, and if you tap it, you can have the map automatically rotate as you rotate your device. With "Sensors" disabled, tapping the compass just points the map in one specific direction, regardless of the phone's orientation (as you would probably expect). I also found that disabling sensors means that AR Mode does not work at all, just rendering as a black screen. However, despite these clear differences with the permission revoked, I cannot see any notifications about Pokémon GO trying to access sensors and being denied.
Just in case this was some weirdness with Pokémon GO specifically, I installed https://f-droid.org/packages/com.bobek.compass/ . With sensors disabled, the app itself displays the following error message:
An error occurred.
Message: Sensor not available
Code: MAGNETIC_FIELD_SENSOR_FAILED
I don't receive any system-level notification however, and this appears to be an error generated by the app itself.
Is this notification about apps being unable to use sensors being displayed somewhere that I'm not looking? The website mentions that this notification can be disabled, although from searching Settings for "Sensors" I don't see any options to toggle this notification, so I'm unable to find any menu to enable it, in case I have somehow accidentally disabled it.