These may need to be reverted. Introducing an arch-specific dependency means the metapackage itself becomes arch-specific, which means it requires cross-builds to work on any architecture other than amd64. We might be able to avoid this somehow (maybe cross builds can be made to work without too much pain, or maybe we can figure out how to tell our build tools that the package contains no arch-specific code and therefore doesn’t need a true cross-build, but if there isn’t some way to do that, we may need to go back to requiring virtualbox-guest-additions-iso even on architectures where it doesn’t make sense.
EDIT: dpkg-buildpackage -i -us -uc -b --host-arch arm64 seems to work on an amd64 box here. Maybe this isn’t impossible to unravel…