It seems to only be used within the physical-plan create and adds quite a bit of API surface area: https://github.com/search?q=repo%3Aapache/datafusion%20JoinHashMapType&type=code

I could not find any public usage of it searching GitHub.

I pinged the original PR authors in 2156dde#r173199638.

cc @berkaysynnada @ozankabak