What is that feathered uneven edge on the LeRoy Neiman serigraph?

That is known as a Deckle Edge.

The feathery uneven edge which is the result of the natural run-off of wet pulp when making handmade paper.

The rough uneven edges naturally present in early handmade art and papers, generally observed in art, pictures, newspapers and magazines printed with pulp or woven paper before about 1830. The great majority of these original edges were lost to trimming when the papers were bound into volumes. Uncut art, newspapers and paper pulp magazines retain these original, irregular edges, and are preferred by the most demanding specialist collectors.