Quotation from the official FAQ:
Updating to Skype (version 8) for Windows Desktop is not mandatory at this time, but support for Skype (version 7) for Windows Desktop will be ending in September 2018. We want to ensure that you have the smoothest upgrade experience and are able to upgrade when it is most convenient for you. Upgrade today so that you don’t experience any interruptions in Skype service in the future.
This update is currently only available on Windows 7 and 8. After September 2018 Skype (version 7) for Windows desktop will no longer be supported on versions of Windows that are older that Windows 7. Users on these platforms are encouraged to upgrade to the latest version of Windows.
Updated 16.07.2018 (quotation from the official blog):
Today, we are rolling out an updated version of Skype (version 8.0) for desktop that will replace Skype version 7.0 (also known as Skype classic).
We are encouraging everyone to upgrade now to avoid any inconvenience as only Skype version 8.0 will work after September 1, 2018. As we roll out improvements, there comes a time when we must shut down older services and application versions.
Updated 06.08.2018 (quotation from the official forum):
Based on customer feedback, we are extending support for Skype 7 (Skype classic) for some time. Our customers can continue to use Skype classic until then.
Thanks for all your comments - we are listening. We are working to bring all the features you've asked for into Skype 8.
Watch this space.