It is not possible to restore a previous version when an owner or expert has left the company and they have been deleted from the system. The process has a current owner and expert.
I have recently had a need to restore a previous version when I accidentally did a bulk change of ownership which published processes in progress.
This appears to be more of a bug than a new feature, so I'll pass it on to our team.
We are planning on making changes to ownership transfers so they do not require publishing.