I wasn't lying before. SkyDemon really doesn't know you're flying on a flightplan. The fact you have previously filed a VFR flightplan for a route (on that device, or any other) is not known by any particular device logged in to the same SkyDemon account.

Our planning warnings and suggestions are very different to the new class of alert you are proposing. I just don't see how and where it would fit in. You may know some other people who think it's a good idea, but unfortunately nobody else has ever suggested this. If they do, we can start thinking about the best means to make such a thing accessible.