I do use the flight plan and trips sections to plan flights, but never actually filed (or activated for that matter), but I was getting the notifications even if there were no upcoming flights in queue and only for that specific aircraft that had ATC Proposed Route Notifications toggled on...
This post came up from a google search cause it was bugging the hell out of me too! Couldn’t find a solution anywhere but I just accidentally came across the setting that prompts these notifications.
In the Garmin Pilot app, go into Settings>Aircraft. Select an aircraft. Then scroll till you...