As of Unity 6.3, the Magic Leap (x86_64) build target is now limited to existing projects only. You can still open and build projects that already target Magic Leap, but new projects can no longer be created using this build target in Unity 6.3 or later versions. To create new Magic Leap projects, please use Unity 6.2 or earlier.
If you have questions about how this change affects your current or future projects, feel free to post below.