What to do with an underground pool outlet now that the pool is gone
The only safe thing to do is to disconnect the wires at the other end. That could be:


The main panel
A subpanel
A junction box - e.g., the wires might come out of the ground next to the house and go into a junction box to splice to another set of wires that go to the main panel


If you disconnect any place other than a panel (which obviously would be accessible), the remaining location (e.g., another junction box) must be accessible. Cap any remaining wires with wire nuts so they can't touch anything else in the panel or box and cause a problem.