This is pretty normal since the 60's. People just didn't call it "poly".

Think about the tv show Friends: all the men and women paired off at different times. Same thing for How I met your mother or Rent. This kind of thing is very commonly in the restaurant business, resort staff, among frats / sororities, and of course FTX/Ameida (ba dum tss).

The difference with GenZ is this behavior is being explicitly codified into this monstrosity of all the worst aspects of HR/pop-psychiatry/family-law. It was always messy before we started calling it a polycule, but alleged solution seems (to this cranky old man) to be worse than letting it play out.