AMD has been transitioning their kernel driver from the closed-source fglrx to the open-source AMDGPU driver that was announced last year. This forms the base that both closed and open user-mode drivers will utilize. For the upcoming Ubuntu 16.04 LTS, Canonical has decided to deprecate fglrx and remove it from the system upon upgrade. Users can then choose to install an AMDGPU-based one, or reinstall the Radeon driver. That will need to be done without Canonical's support, though.
It makes sense that they would choose Ubuntu 16.04 to pull the plug. This is the version that Canonical will be maintaining for the next five years, which could give a headache when AMD has spent the last year trying to get rid of it. AMDGPU is a much safer target as the years roll forward. On the other hand, GPUs prior to Fiji will not have the luxury of choosing, because AMD still hasn't announced AMDGPU for
GDC (Update March 9th @ 6pm: Fixed typo) GCN 1.0 and 1.1.