In an announcement on its support page, Microsoft writes, “Effective May 20, 2018, the Windows Phone apps for Skype for Business, Microsoft Teams, and Yammer will be retired. Starting on that date, these apps will no longer be available for download from Microsoft Store, and Microsoft will no longer support them.”
Microsoft also made it a point to say that they are still supporting Windows 10 Mobile as a platform in general, it’s just that these apps will no longer be supported and that the company isn’t focused on building new features for the platform for now. They also point out that while support for these apps on Windows 10 Mobile will be coming to an end, that’s where it ends and that it will not affect those apps on other platforms.
According to Microsoft, “We will continue to invest in the Skype for Business, Microsoft Teams, and Yammer mobile apps for iOS and Android, and also in desktop apps for Windows and Mac.”