Microsoft is ending support for older version Operating Systems for Skype. The Application has gradually evolved to support array of new features. It is but obvious that, older platforms cannot support these new innovative additions.
Maybe Microsoft is not keen on supporting so many OS versions out there. Just recently Skype has begun transition from P2P based architecture to a cloud based. This has allowed Microsoft to include Video Calling, improved account security etc.
If you are using Skype on desktop and running on Windows XP SP3 (32-bit) than make sure that you are using Internet Explorer 8 or above. Users with Windows 7 SP1 will have to upgrade Internet Explorer to V11. Windows RT will no longer be support from October 2016.
Mac OS X 10.6 Snow Leopard wont be supported as well. Skype 7.0 for Mac client will only work for macOS 10.9 mavericks.. If you are running on Windows Phone 8 or 8.1, make sure to upgrade to Windows 10 Mobile by October as the former versions will be obsolete. Android Phone users should have Android OS 4.0.3 version or above.