In the case of IG-88 it should be noted that he is of a line of assassin droids so one could appear and it just be another one of them. As to reusing props, I'm pretty sure that IG-88's head is even a case of reusing props from the bar in A New Hope.

However according to a brief internet search, IG-88 is still active post-Endor relatively close to TFA so maybe he's still around.