I was talking about Sinnoh/Hisui, but the other region I was thinking of was Kitikami (from Scarlet/Violet’s Teal Mask DLC), as white-stripe Basculin seem to show up in the latter’s waterways.

But yeah, migrations between Hisui and Johto might still have happened, considering that Sinnoh still has Sneasel in the modern day, but only the Johtonian subspecies.