This page on Microsoft Learn will get you what you need:
https://learn.microsoft.com/en-us/skypeforbusiness/software-updates
It is a list of all the Skype For Business client updates including 2015.
Hope this helps!
Rich
Community Manager
Microsoft Q&A
Hey @JimmyYang-MSFT - just a heads up, as I shared yesterday, download links for those older clients are available at:
https://learn.microsoft.com/en-us/skypeforbusiness/software-updates
Hope this helps.
Rich
Community Manager
Microsoft Q&A
@Josh Pata
Haven't received your message for a long time.
Do you have any further issue on this topic?