See https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=b30a43ac7132cdda833ac4b13dd1ebd35ace14b7

Dave Airlie:
There was a nouveau DDX that relied on legacy context ioctls to work,
but we fixed it years ago, give distros that have a modern DDX the
option to break the uAPI and close the mess of holes that legacy
context support is.