As of May 20, 2018, Microsoft will no longer make these apps available for download from the Microsoft Store and will no longer support them.
The Skype for Business and Yammer apps "may continue to work, but we can't provide any guarantees," said officials in a Microsoft support article. "These apps will no longer receive updates or support."
At the same time, the Microsoft Teams app for Windows Phone "will no longer work, and users will receive an error message when they try to connect," the article said.
Microsoft is continuing to support the Skype for Business mobile apps for iOS and Android, as well as the versions for desktop Windows and Mac. Microsoft's guidance for Windows Phone users is to access Skype for Business, Teams, and Yammer via their web browser.
In related news, Microsoft also noted last week that it will be removing a number of Yammer features in May 2018. The company is getting rid of Yammer's native org chart feature; its recent activity module on the home page; and the Yammer desktop notifier applicaiton.
Microsoft is advising Yammer users who wanted to continue to take advantage of these features to look at alternatives that are built into Office 365.