What is the maximum altitude ATC would deal with?
There are many examples of aircraft with high service ceilings. While most commercial aircraft have service ceilings of FL410 and rarely fly even that high, many business jets have a service ceiling of FL510, such as the Dassault Falcon 7X, Gulfstream G650, and Bombardier Global Express.

That being said, military jets can fly higher and will be talking to ATC while in their airspace. Fighter jets are capable of high altitude flight, and planes like the U-2 and Global Hawk still fly up there as well.

In the US, all aircraft from FL180 to FL600 must be in contact with ATC, but above this they are not required to. ATC will still provide services when able though.