You need an explicit marker for compat with legacy code. The issue defines unspecified as "a null may occur, but we don't know whether its presence is deliberate".

Explicit nullability fixes the missing information in older code. There could be other ways to mark opting into strict nullability through metadata at the class, module, package or VM level; but fine grained markers are the less disruptive for gradual migration. In particular this keeps the information local instead of relying on an ambient context.