Before the truncation, PA 100 used to exit before the spur started (just south of the US 30 "bypass") as a continuation of Pottstown Pike, head into and through West Chester, then multiplex with PA 52 before breaking off on Creek Road, to eventually join with DE 100. You can trace it by following SR 3100.
So..it actually was a spur. It was never signed as a spur. The exit on US 202 used to read TO PA 100 North / TO US 30 West. I don't think that stretch has an official name so PA 100 Spur is a fill-in-the-blank.