I just hit this too. This issue makes crossbuild-essential-arch useless. doko: is the concern in your note on this bug report still valid? If so, what specifically prevents this migration to testing? Is there some tool in the infrastructure that's not sufficiently Multi-Arch aware? Which tool is it?
Since stretch is now frozen, can we please fix this in sid now, and agree to prioritize fixing whatever is still broken in the tooling? I can work on it if you point out the culprit.
