[ajb]

I don't know about recently, but people were asking about them from the first announcement in 2009; and got the answer that they were "under consideration"

To be fair, it's a significant advantage of go that they have been strict about keeping it's feature set small.

Edited to add:

There is this 2023 proposal from Ian Lance Taylor (on the go team) https://github.com/golang/go/issues/57644 But it makes all sum types include nil, which seems suboptimal

[pstuart]

Well, he advocated for and eventually got generics, so it could happen here too.