Not only is it legal, but altitudes above 60,000 feet are class E airspace.
This means that if you are lucky enough to have an aircraft that flies that high, once you get there you can theoretically call ATC, cancel your flight plan and continue VFR above flight level 600 without ATC oversight.
At that point, you’re on your own and out of the system.
You are literally beyond the limits of human civilization.
You are where the sky turns from blue to black.
Congratulations, you are in a place where very few civilians have been.
If you have a civil aircraft that allows you to do that, I envy you very much.
Photo taken from a U2 spy plane, ~70,000 feet, courtesy of the Daily Mail. It’s not a civilian plane.