This is a new property in MW 1.28. I'm still not sure what it does though.
Activity
Choco31415 commentedon Oct 13, 2016
Apparently this field returns, for a valid but non-created username, if it can be created. Link
Choco31415 commentedon Oct 13, 2016
I'll remove this from the milestone for now since it will most likely be added in 1.28.