If you’re self-hosted you can do it from the rails console. If you’re hosted, you can ask support.
There might be some hints over in Administrative Bulk Operations.
Something like
t=Topic.all.with_deleted.where("deleted_at is not null").where(user_id: 123)
t.update_all(deleted_at: nil, deleted_by_id: nil)
But I think you will then need to run something else to update some other tables.