Microsoft has now confirmed it has removed the built-in FM Radio app inside Windows 10 Mobile from its development branch builds and it will be cut from the OS going forward.

FM Radio

Microsoft quietly removed the FM Radio app in the Windows 10 Mobile preview build 14322 that was released to Insiders in mid-April. More recently, a member of the Windows Insider Engineering team, answered a question about its disappearance with this response:

Microsoft has not indicated the reason for its removal of the FM Radio app, but as previously noted, there are third-party apps in the Windows Store that offer access to FM radio stations.