WASHINGTON (AP) — The Federal Aviation Administration said Wednesday that it was taking the extraordinary step of reducing air traffic by 10% across 40 “high-volume” markets beginning Friday morning to maintain travel safety as air traffic controllers exhibit signs of strain during the ongoing government shutdown.

The cutback stands to impact thousands of flights nationwide because the FAA directs more than 44,000 flights daily, including commercial passenger flights, cargo planes and private aircraft. The agency didn’t immediately identify which airports or cities will be affected but said the restrictions would remain in place as long as necessary.

This article has a list, but apparently it's not final.

In a letter to employees, United CEO Scott Kirby promised to focus the cuts on regional routes and flights that don’t travel between hubs. He said the airline will try to reschedule customers when possible and will also offer refunds to anyone who doesn’t want to fly during this time, even if their flight isn’t canceled.

...

The cuts could represent as many as 1,800 flights and upward of 268,000 seats combined, according to an estimate by aviation analytics firm Cirium. For example, O’Hare International Airport in Chicago could see 121 of its 1,212 flights currently scheduled for Friday cut if the FAA distributes the reductions equally among impacted airports, Cirium said.

I assume there are legal reasons why the airlines can't pay the salaries of the air traffic controllers. If they could, I'm sure they would because this will cost them millions.