Good question. Back in the day when it was still called the Union Plaza, the hotel pool was in the spot later occupied by the Center Stage restaurant, followed by a string of others, and was, if our memory serves us correctly, in the shape of a cocktail glass.
The pool later moved to the rooftop, and until a few of years ago hosted fun summer parties (see photo below, from 2008). It was then closed, along with the rest of the property, for the major renovation job that just took place.
A call to the concierge today confirmed that the pool is still closed and will remain so until, they hope, no longer than the beginning of pool season next year, while a major upgrade takes place. Details were sketchy, but they're building bungalows and a full-service bar up there. Should be pretty cool when it reopens.