This issue tracks the deprecation of the select-validator option. The validator will be internal by default. The only way to override this is to provide an environment variable when invoking dxc.exe. There is no longer any use of this option, and it is not officially documented anywhere.
This is compliant with the soon-to-be design proposal here:
https://github.com/microsoft/hlsl-specs/pull/499/files#diff-37d48932bf2b3d27214cfe1f7e46168df5309cdb9a56cf16583b7a78e106f600