I searched the roadmap but did not find any plan of update of Skype for Business 2015 control panel.
Considering Microsoft is not planning to take any specific action to terminate Silverlight applications. It recommends you try to plan migration from Skype for Business 2015 Server to Skype for Business 2019. The following link shows the process of a pool migration to Skype for Business Server 2019, you can refer to:
https://learn.microsoft.com/en-us/skypeforbusiness/migration/migration-phases
If the response is helpful, please click "Accept Answer" and upvote it.
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.