The general consensus among industry insiders and journalists is that it will be exclusive to the Xbox ecosystem.
We may well see it on Switch or whatever Nintendo console is out, if it can run it, but I have no doubt all future Bethesda titles will be Xbox only.
Otherwise, why buy them? A huge part of the PS4 success was down to platform exclusives, makes no sense to shell out billions and not make them exclusive.
MS might not make as much from the software sales but will drive console sales, additional hardware, people to join the ecosystem and Gamepass subscriptions.