https://en.wikipedia.org/wiki/Tagged_union#1960s (and so on in the rest of the article; you could make the argument that being able to express a sum type and using option/result pervasively are different things; I don't know much about early ML but it was from '73...)
ML has roots that go back a long ways, but it wasn't developed as a general-purpose programming language (for use outside theorem provers) until the '80s, and I'd consider it properly "released" to the general public as something intended for real use only in the 1990s, with the publication of the Standard ML definition (1990) the release of OCaml (1996).
