As mentioned in MDL-48969 (https://tracker.moodle.org/browse/MDL-48969?focusedCommentId=339708&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-339708)
It should be possible to allow a plugin to be able to "claim" ownership of backups from other plugins. This would allow depreciated plugins to be fully removed instead of requiring a "stub" to be left behind for the purposes of restore.