Microsoft announced that it's updating Skype today, promising "improved quality and better performance", something that the company has promised numerous times. The firm also said that in order to provide you with the latest experience, it will be shutting down older versions of Skype.
The versions that Microsoft noted are all about a year old. 7.16 for Windows was released back in December, 2015, and anything newer than that should be fine. If you're on a Mac, you'll need a version that's better than 7.18, the latest of which was offered in February, 2016. If you're still on one of these versions, you should check for updates under the 'Help' tab.
This is likely part of Skype's transition to the cloud, which also caused the company to end support for other platforms, such as Windows Phone 8/8.1 and Windows RT. The service is also slated to stop working for those platforms in early 2017, although Microsoft hasn't provided a specific date just yet. We've reached out to the company for clarification and we'll let you know when we hear something.