I found this image on Ancestry.com for a John Scott Robb from South Fayette, Allegheny, Pennsylvania, born Feb 4, 1860. I am assuming this is the same man as the person who umpired an American Association game in Pittsburgh in 1886. Retrosheet lists John S. Robb as being born in 1860 in Allegheny County, Pennsylvania with no death information.
The man I found on Ancestry died on March 8, 1939 in Pittsburgh. I don't know if SABR can verify that these two are the same men, but I think it has to be a match.