Hmm, this is an interesting idea. Since this is a gopls feature request, I'll transfer to the Go issue tracker.

Something @adonovan and I have discussed is the extension to 'Find References' to find all assignments of a variable (something I often want). This could be part of that analysis.

The current Hover functionality does almost exactly that: if you position the cursor within an element of a function's return type, it highlights all the corresponding return operands within the function body. And, conversely, if you position the cursor in one of the return operands, it highlights all the others and the corresponding element of the function's return type. It works whether the return variables are named or not.

There are other places in the grammar that make an implicit reference without an identifier, the most obvious one being that S{x} is an implicit reference to a field of struct S. But things get fuzzy: if return 1 is an assignment to a named result variable, is f(1) not then an assignment to the parameter variable? (And what about aliasing? Given ptr := &x, is *ptr = 1 an assignment to x?)

Making FindAllReferences return implicit references could be quite confusing, and I'm not convinced it's worth doing given that the hover feature I mentioned above provides a simple solution to the immediate problem.