This change was ported from types2, where it was needed IIRC to support -lang. It being exported was incidental, and for that reason in particular perhaps we should unexport this field and discuss it for 1.18. @griesemer, what do you think?
Some features (and thus error messages) depend on the Go version. As long as we're ok with go/types running against the latest language version we don't need to expose this.
To clarify, I think this is a useful feature, and we should expose it eventually. But the fact that it was exposed in go/types for 1.17 was incidental. The real reason it was added was for consistency with types2 (and because it has value of course). At the time we weren't thinking about 1.17.
If we think it's minor enough not to require a proposal, we can leave it in, but I certainly don't want it to be perceived as bypassing the proposal process. Since no one is clamoring for this change, unexporting the field and making a brief proposal at the start of the 1.18 cycle seems like a fine option. I feel roughly the same about #46298, though I think that change is even more minor.
I defer to both of you, as members of the proposal committee, for what is OK.