User Configuration > Administrative Templates > Skype for Business 2016 > Microsoft Lync Feature Policies
User
HKCU\software\policies\microsoft\office\16.0\lync
Specify a semicolon separated list of server version names, e.g. RTC/2.9;RTC/3.0;RTC/4.0, to which Microsoft Lync allows logon in addition to the server versions that are supported by default. Space character is treated as part of the version string.