Is it safe to touch one terminal of a floating 20kV 1A PSU?
Those of who who remember nylon shirts should remember that you got an electric shock every time you put one on or took one off. If you switched the lights down and closed the curtains you could even see little blue flashes as you wrestled with the electrostatic forces causing the shirt to cling to the skin. Thousands of volts were generated but the energy was low due to the capacitance being low.

If you have a 20 kV supply that is floating and powered from a battery (i.e. it is galvanically isolated) then there will be zero leakage to ground but there still could be some tens or even a hundred pico farads of capacitance that is charged up to 20 kV. I might be tempted to touch one of the floating leads if a nurse was on hand.

If on the other hand you have a 20 kV supply that is floating but powered via an isolating mains transformer, then capacitance to ground could be hundreds of pico farads and may even reach several nano farads. I wouldn't want to touch one of these.