On https://learn.microsoft.com/en-us/windows/win32/direct3d11/overviews-direct3d-11-devices-downlevel-intro I see the feature "Map Default Buffer" which is an optional feature since feature level 11_0. What can I make with this? Which advantages provide this feature? Which GPUs support it?
This optional feature was introduced with Direct3D 11.2. Most Feature Level 11.0 or better devices with WDDM 1.3 or later drivers support it, although you still have to check that it's supported with CheckFeatureSupport. That means you should have a fallback path that doesn't use it if needed.
This feature was covered at the 2013 Build event. The video is on Channel 9.
It is designed to improve texture streaming scenarios allowing you to use Map DISCARD style updates directly on a DEFAULT resource without having to copy it to a staging resource first.