Hatmatter
Laws of Mordenkainen, Elminster, & Fistandantilus
Well, now that Theros is out on D&D Beyond, it has this to say about Theros' place in the multiverse:
"The world of Theros, as its inhabitants understand, includes three realms: the mortal world, the divine realm of Nyx, and the Underworld. They are three distinct planes of existence, tucked into their own pocket of the multiverse and shielded from the rest of the cosmos by the power of the gods."
So, like Eberron, it's technically part of the D&D multiverse, but cut off and inaccessible unless the DM wills it otherwise.
Cool! Thanks!