AMD Rebrands CrossFire As Just mGPU
The mGPU term is just short for multi-GPU. They are dropping their multi-GPU "CrossFire" branding due to a shift in approach in how games make use of multiple GPUs. Traditional CrossFire, similar to NVIDIA's SLI, has relied upon game profiles for leveraging rendering across multiple graphics cards. That was with Direct3D 11 and prior (and OpenGL) while with Direct3D 12, the responsibility of multi-GPU handling is punted off to the developer with the D3D12 APIs for implicit and explicit multi-adapter support. As such, it's really not "CrossFire" anymore but just mGPU for future games.
Vulkan is similar to Direct3D 12 in that the responsibility of multi-GPU handling is left to the game developer via extensions for managing resources and rendering across multiple GPUs.
When it comes to CrossFire support on Linux, the RadeonSI+AMDGPU or AMDGPU-PRO stacks have not supported it in the traditional sense. With the former Catalyst/fglrx driver stack there was OpenGL CrossFire support, but it was largely useless and not many game profiles supported on Linux and so the benefits were minimal. But with the Vulkan support, there is said support, but it's a matter of when game developers begin adding multi-GPU Vulkan support to their renderers...
There is nothing blocking anyone from adding OpenGL CrossFire support to the open-source AMD Linux graphics stack, but it will probably never come. AMD obviously doesn't have motivation now to add the support with Vulkan being around, the AMD developers are busy as-is with a ton of other projects, and it just doesn't make too much sense. Even NVIDIA SLI for Linux gaming isn't too practical.
Khronos added its initial multi-device support to Vulkan earlier this year with their v1.0.42 GDC update. These extensions are for the external memory, device groups, device group creation, etc.
It could be a good question for Linux games when comes the multi-GPU benefits given there aren't many Vulkan titles right now and game developers are more concerned about seeing the games work across the Linux drivers and performant than seeing it working for the small percentage of Linux gamers using multiple GPUs.