Hello Peter,
The type of problem that you raised is the exact reason why I proposed another org. Why would you have a source dep on something that is no longer maintained? That is really bad for the active project because the raw unmaintained repository is included in something active. What if there are issues with building the unmaintained code? Who will fix it? Nobody should touch the archived repository!
The proper course of action for the scenario you described is to have the active project drop the dependency on the archived project. And the archived project should have had a release prior to being archived. The active project should have depended on the officially released version of the archived project and drop it as soon as possible.
If we set aside srcdep scenario, do you have any other objections to a new org for archived repositories?