I'm watching ADSBexchange too.
It's eerie seeing the total number of aircraft tracked decrease. I put on a filter for 1000' to 80000', to exclude aircraft on the ground, and the number was around 12K when I started watching. It's down to just over 9.4K now. Wild.
Edit: 22 minutes later and the count is down to 8.4K.
Now under 8k
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