Since wgSessionsInObjecgtCache is deprecated, it would stand to reason that this should also be deprecated...right?!
Topic on Manual talk:$wgObjectCacheSessionExpiry
@Ahancie Reported this upstream (Phabricator): https://phabricator.wikimedia.org/T158829
Work on deprecating the variable will continue from there :-) If you haven't already created your account, read this useful guide and subscribe to the ticket! Subscribing will give you notifications on the tickets whenever it's updated (e.g someone comments on it)