Well, majority of top 15 at least are still from USA, and most of them, no matter where from, train in USA, so I would slow the roll, lol.
But there's definitely some shift going on lately.
Błachowicz have been training pretty much 100% in Warsaw for a while now. And few other places in Poland.
Adesanya and Volkanovski obviously proudly represent City Kiwiboxing.
Pert Yan is also training outside of USA for most part, AFAIU
Khabib and Figueiredo train in USA a lot, though they're obviously products of their homeland environment.
Stipe and Kamaru are pretty much 100% product of USA, even though Usman've spent large portion of his childhood in Nigeria.
Nunes is Bhazil, now lives and trains pretty much exclusively in Florida?
Schevchenko is quite a roller-coaster, Ukrainian from Kazakhstan in Peru, does some training in USA.
Zhang is pretty much completely USA free so far?