I suspect this is WAI. See the discussion in #28622, in particular Alan's comment at #28622 (comment).

@alandonovan Is there an existing shadow checker somewhere that uses the new framework that people can use?

Marking for 1.12 because if nothing else this needs to be mentioned in the release notes.

Is there an existing shadow checker somewhere that uses the new framework that people can use?

Yes, see go help vet:

$ go help vet 
usage: go vet [-n] [-x] [-vettool prog] [build flags] [vet flags] [packages]

Vet runs the Go vet command on the packages named by the import paths.
...
The -vettool=prog flag selects a different analysis tool with alternative
or additional checks.
For example, the 'shadow' analyzer can be built and run using these commands:

  go install golang.org/x/tools/go/analysis/passes/shadow/cmd/shadow
  go vet -vettool=$(which shadow)

...
See also: go fmt, go fix.

The reason this checker is not included in the new vet is that it was the sole "experimental" checker, and I didn't think it warranted the introduction of the "experimental" concept to the new API. If we can improve the heuristics used by 'shadow' to the point where it would no longer be considered experimental then there's no reason not to add it back to the core vet suite.

Change https://golang.org/cl/154584 mentions this issue: doc: explain how to use "go vet -shadow"

Quick reminder to only comment if you have an idea or constructive feedback. A new issue would probably be better too, as this issue is closed and isn't the best place to discuss.

In any case, personal opinions and criticism don't add to the thread.