We have made an application which uses the Lync 2013 Server SDK to monitor interactions.
The IT department asked me if they can allow an upgrade to Skype for Business Server 2015. There doesn't seem to be a Server SDK for SfB 2015.
Will my application which uses Lync 2013 Server SDK still work?
Yes: https://msdn.microsoft.com/en-us/skype/skypedeveloperplatform
Take a look at the table at the end of the topic.