If I am following a route in my inreach, and I take a deviation from the trail, is there any way to get a notification warning me of that?
I don't have an active legacy device, so I can't experiment. From what I recall, the answer is no. IIRC, the Explorer+ routing behavior is to direct you (in a straight line) to the nearest route point "ahead" of you on the route. I THINK (but no longer recall for sure) that the device will switch from one route point to another if your movement causes a different point ahead on the route to become "closest to me at the present time." The only way to tell if you are not actually following the route is to look on the map on the device.
Do not take my word for any of that. It's worth experimenting before you take it into the back country.