To be clear, there are no in-tree filters that rely on RequestSpec.user_id, but people have out of tree filters that could have been using filter_properties['instance_properties']['user_id'] and the change in mitaka regressed them. It also seems OK to have RequestSpec.user_id since we have RequestSpec.project_id, and I could see it being useful to know which user created an instance when it's being moved if another user in the same project is moving the instance.
To be clear, there are no in-tree filters that rely on RequestSpec. user_id, but people have out of tree filters that could have been using filter_ properties[ 'instance_ properties' ]['user_ id'] and the change in mitaka regressed them. It also seems OK to have RequestSpec.user_id since we have RequestSpec. project_ id, and I could see it being useful to know which user created an instance when it's being moved if another user in the same project is moving the instance.