Windows Phone had the app gap problem from the day one. Even today the whole situation has not improved much. It is only recently that Instagram launched its official app for Windows 10 Mobile which made everyone believe that good days are coming. But the situation has indeed worsened with the popular HERE navigation apps leaving Microsoft's OS.
HERE apps were the prominent ones and one of the reasons people still used the Windows Phone. The HERE Maps which was originally developed and integrated into Windows Phone 8 was made to work on Windows 10 Mobile with a workaround.
The company would need to develop its navigation apps from the scratch to continue supporting Windows 10 Mobile which it doesn't want to do. So the company has just decided to stop the development. It is not a surprising move as the OS has failed to grow as expected to give developers any reason to consider it.
As a result, all HERE apps will be removed from Windows store on March 29. If you already have HERE apps on your phone, they will stop working from June 30. However, HERE apps on Windows Phone 8 will continue to work but won't get new features. So either way, the users will be at the loss. HERE has advised users to use the Windows Maps which is developed by Microsoft itself.
Well, Microsoft has a pile of things to do to get users, developers and everyone else to believe in its mobile OS.