Statement of Microsoft:

“On February 21, 2018, we announced the deprecation of the Windows Phone app for Dynamics 365 (online), version 9.0. On May 22, 2018, the Windows Phone app will no longer be supported for Dynamics 365 (online), version 9.0. Support for the app on Windows tablets and PCs remains unchanged. The Windows Phone app will continue to be available in the store and will be supported for Dynamics 365, (version 8.2) and earlier supported versions. If you wish to continue to use the Windows Phone app, you should not update to Dynamics 365, version 9.0 as the Windows Phone app will not be supported with version 9.x….”

Find more in Microsoft docs article: User Guide (Dynamics 365 for phones and tablets)