Scripture doesn't say that that was the end, does it?
No, it was not said, “There are no more”. If more “keys to the Kingdom were used then the question is what and where and by whom?
Calling something a “key to the kingdom” would indicate those locked out in some sense were now able to enter the kingdom.
Is there some situation recorded in Scripture that would unambiguously indicate Peter was using other “keys” beyond those three instances recorded in Acts?