Digging a bit further, the original change was in mid-2018 at https://go-review.googlesource.com/c/go/+/122878/, presumably to better support foo.test packages for #25949. Unfortunately, the commit message doesn't really clarify much for me.

Ah, it seems like ImportMap is used for vendored packages in the standard library, but not in the current module:

$ go list -json vendor/golang.org/x/text/secure/bidirule | grep -A3 ImportMap
	"ImportMap": {
		"golang.org/x/text/transform": "vendor/golang.org/x/text/transform",
		"golang.org/x/text/unicode/bidi": "vendor/golang.org/x/text/unicode/bidi"
	},

This seems to be what I was running into. It's still unclear to me why vendored packages only appear in ImportMap in this case, but not others.

ImportMap used to be more significant: in GOPATH mode with per-package vendor directories, it reported the mapping of import paths to vendor trees.

In module mode it is indeed less useful, because we now have only one copy of each module dependency (instead of one per vendor subtree). However, it will probably become a bit more meaningful again once we get replacements straightened out (#26904, #30831).

Thanks; at least this confirms that my being confused at the docs is somewhat justified :) I'm happy to have this issue in the backlog until those two issues are fixed. Once they are, perhaps the docs can be clarified with an example or two.

On the topic of confusion: #46462