ML had "it" in 1973!
... if "it" is even a thing. I would say that it's not, rather implicit nullability is the thing. It's just not something you'd do in a typed language unless you really specifically chose it. That's how Hoare could claim it was his billion dollar mistake. Implicit nullability was a thing he (and subsequent language designers) opted in to.
I know OCaml[1] has it ('93) and Haskell[2] ('90). Also the claim that implicit nullability is a mistake was made in 2009[3] (around the same time Go was released).
Given the seniority of the designers of Go, I expect they had knowledge of this.
https://en.wikipedia.org/wiki/OCaml
https://en.wikipedia.org/wiki/Haskell_(programming_language)
https://en.wikipedia.org/wiki/Tony_Hoare