KD joining Steph is unprecedented, but not because he left someone of Russ’s caliber; that has been done before, and it was done by one of the three superstars you cite in your parenthetical.
When Shaq left Orlando at the end of the ‘95–’96 season, there was one Orlando Magic player on that season’s All-NBA First Team (and the season before). His name: Penny Hardaway. Shaq was the 3rd Team Center in 95–96, and All-NBA Second Team in 94–95. We can argue Penny’s Top 5 status during this period of time, but he was certainly in the conversation.