As a TU, I cannot disown without knocking off co-maintainers
As an end user I used to be able to add co-maintainers to a project, then if I wanted to bow out I could disown it and it would promote one of the co-maintainers to be the maintainer.
As a TU that process goes sideways. Now if I disown a project all of the other co-maintainers are removed as well! Oops, that's not what I wanted to have happen to the package
As a TU I can add the co-maintainers back without adopting it, but that still leaves the project marked as maintained by "None" and up for adoption.
I suggest the same behavior as regular users see would be best, in the event it is not "self" then just disown by the current maintainer and promote the next co-maintainer. To completely clear a project you might have to do it a couple times, but that's better than clearing everybody when what you need is one.