The privilege is proportional to the show's success. I was not into kpop back then but I feel Produce 48 was quite a popular show, wasn't it?
More popular than GP999 for example. In the case of GP999, I don't expect a lot of members to benefit from "Produce" privilege after Kep1er is done. Too many fans got soured because of past controversies, plus new disgruntled fans this time over dubious voting rules.
So I don't know, maybe the PD48/IZ*ONE privilege is the only actual privilege among the Produce series. But if it's real, it's real.