If you haven’t seen it, there’s an exhibit about 9/11 at the Smithsonian Air and Space Museum in DC that shows the airspace in the lead up to and just after the attacks. If you find the traffic changes now interesting, I think you’ll find that interesting as well. It’s possible it’s online if you do some googling

Now under 8k

FL400. 40 AGL is doable for almost all airliners ;)

I saw several flights taking off actually. Like this one from EWR. https://globe.adsbexchange.com/?icao=394a08

It doesn't look like the stop really applied to VFR flights, so I wouldn't expect an old Glasair taking off from a municipal airport to be affected.

I live in the general area of you too. Nice to know that there are Detroiters on here.

It could be the flights that are heading directly away from US airspace over the atlantic to free airport space for other planes?

My view on ADSBexchange is centered on CMH, zoomed out to Washington D.C. to the east, and filtered by 18K up I've got 142 aircraft.

This tracks with what I has seem elsewhere. I don't have the details handy now, but the communication system used for long range handoffs for flights above 18,000 ft seems to be what is impacted. Keeping planes below that deck would keep them bouncing more frequently between shorter range towers but at least still tracked and in communication.