Starting March 1, 2017, Microsoft is going to discontinue certain older Skype versions. For Windows, you need at least Skype 7.16, for macOS, you'll need Skype 7.18 and for Linux, you will probably need the newest alpha version of Skype.
The Redmond giant is about to drop server-side support all older Skype clients which use the peer-to-peer protocol to establish a connection between clients.
This will affect Skype for Windows versions below 7.16 and Skype for macOS versions below 7.18.
Microsoft is moving all Skype clients from a decentralized P2P architecture to their modern client-server infrastructure which uses Microsoft's servers for all communications.
The announcement doesn't mention Skype 4.3 for Linux, however, it might probably stop working too. Skype for Linux 4.3 is using P2P and it is already not able to communicate with modern Skype clients like the web version of Skype or Skype for Linux Alpha.
For Linux users, Microsoft offers a new Skype app, which is a web app bundled with its own Electron engine. The application is not compatible with older Skype versions, however, it is able to work with all modern releases of the app available for other platforms. As of this writing, its most recent version is 1.17, which still lacks some of the features available in the classic Skype for Linux 4.3 application.
This move can also kill the ability for Skype users to use it on Windows XP. The newest version of the application does not support Windows XP any more. While Windows XP reached its end of support in 2014, many users still have it installed, especially in the corporate sector because there is no substitute for many features and app compatibility of Windows XP which was removed completely in later releases of Windows.
If you are using Skype regularly, ensure that you have upgraded your client software before March 1, 2017.