Microsoft has released an update for Microsoft Skype for Business 2015 (Lync 2013) on April 2, 2019. This update also includes the new Skype for Business client. The version number of this update is 15.0.5127.1000.
MS15-044: Description of the security update for Lync 2013 (Skype for Business): May 12, 2015 See Knowledge Base article 3039779.