LockManagerGroup::singleton is only used in fallback code for older versions and could be hard deprecated.
https://codesearch.wmflabs.org/search/?q=lockmanagergroup%5Cs*%3A%3A%5Cs*singleton&i=fosho&files=&repos=
LockManagerGroup::destroySingleton
https://codesearch.wmflabs.org/search/?q=lockmanagergroup%5Cs*%3A%3A%5Cs*destroySingleton&i=fosho&files=&repos=
Outstanding patch sets for remove deprecated code:
https://gerrit.wikimedia.org/r/#/q/topic:lockmanagergroup+(status:open+OR+status:merged)