Ok so I just started digging for alternatives, thanks to your question.

I had previously simply deactivated network access for Nova (Settings -> Apps -> App Management -> Select Nova -> Data Usage -> Disable Mobile Data, Disable Wi-Fi, and flick the switch for Background Data for good measure.

Now, that will result in a toast on the main screen every time you unlock the phone “Data for Nova7 is disabled, you can activate it in the settings”. This might however be a OnePlus specific error, since searching for it only gives me a result in their product forum.

Nevertheless, I dug around with LogCat Reader and found that the message is displayed by com.android.systemui. Not really surprising, but I wasn’t sure which exact system resource is responsible. So then I went ahead with AppOps and set the “Display Toast” permission to “Ignore”, and voila.

Caveats:

1. You need root to use AppOps
2. You won’t get any toast now (I don’t actually remember seeing any useful ones in over a decade, but there might be something out there)