Toward a fully reproducible Debian

The build-time timestamp updates *on-disk* are not banned, nor discouraged. This is actually the inverse, not doing so would break a ton of stuff. What we do is clamp those timestamps when generating the resulting artifacts to a fixed one that gets increased at controlled times (usually during the package release process, and recorded in the debian/changelog, passed over via the referenced SOURCE_DATE_EPOCH spec). In case of dpkg and Debian that includes at least .deb ar members and its tar archives within, or any other timestamp embedded in the included files by the various other tools, which have been improved to support that spec.

Having a changing timestamp in the resulting files makes no sense, as has been mentioned here. But something I've considered important it still recording the build time because that tracks information that is otherwise more difficult to get now. This information is not really relevant for the generated artifacts. But it helps track events that are initiated externally to the contained build-environment. Say, data-corruption in the filesystem, an accidental file removal, etc.
and that's why we still record it out-of-band (see the Build-Date field in <https://manpages.debian.org/sid/deb-buildinfo>).