Offline messaging is now available after you install the June 2016 cumulative update 6.0.9319.259. This feature is enabled by default if EnableIMAutoArchiving is enabled.
For more information about offline messaging, please see the followings:
Use offline messaging in Skype for Business
Enable or Disable Offline Instant Messaging (IM) in Skype for Business Server 2015
For more information about offline messaging, please see the followings:
Use offline messaging in Skype for Business
Enable or Disable Offline Instant Messaging (IM) in Skype for Business Server 2015