Just to quickly summarise what @cserf @robbarnsley and I discussed today:
To implement this feature the idea is to add a on_delete trigger to the metadata plugin interface. On the deletion of a did, this trigger is then called for each plugin and the plugin itself decides how to best archive the metadata (Or might not do that at all, if not supported by the plugin)

The difficult part is for the column_metadata plugin the data is already moved to the delete_dids table. However, this is mainly done to block the scope, name pair from being re-used in the future and only as a side-effect it archives the column metadata as well. Thus the case needs to be foreseen that a Rucio instance might not want to archive column metadata, but still block a did from being re-used in the future, so this needs to be foreseen in the code.

@cserf will work on a design suggestion on the interface in the next weeks.

Looks like the PR hit a roadblock - are you still working on this @cserf ?

Happy to help if I can. Would be very helpful to have this functionality.