TypeScript _itself_ has a branded primitive string type it uses internally.[1] Dedicated syntax for creating unique subsets of a type that denote a particular refinement is a longstanding ask[2] - and very useful, we've experimented with implementations.[3]

I don't think it has any relation to runtime type checking at all. It's refinement types, [4] or newtypes[5] depending on the details and how you shape it.

[1] https://github.com/microsoft/TypeScript/blob/main/src/compil... [2] https://github.com/microsoft/TypeScript/issues/4895 [3] https://github.com/microsoft/TypeScript/pull/33038 [4] https://en.wikipedia.org/wiki/Refinement_type [5] https://wiki.haskell.org/Newtype