What's the simplest high voltage AC square wave oscillator?
The simplest is a blocking  oscillator, using one transformer, 
a transistor, and a power source (battery?) with a couple of
passive components.
The frequency isn't easy to adjust, the transformer has to be 
customized for the power input and desired output voltage, but it
IS simple.
 typical blocking oscillator
If your output doesn't share a negative or positive rail with the
input power source, a three-winding transformer is used.

While it is simple (in the sense of component count) the design
is subtle and depends on magnetic nonlinearity in the transformer.
It isn't flexible, or inexpensive, in a one-off (because of the
customized transformer).   If you want symmetric square waves,
a two-transistor variant is a possible improvement.

A high-voltage-capable op amp with three resistors and capacitor
is a more common  solution, but 50 mA and 60V are higher power
than most such amplifiers offer.