X-sheets and their usage have also evolved with the way animation is produced.
Modern day animation softwares (like ToonBoom, Adobe Flash, Autodesk Maya) have a tool called Timeline, which in many ways is similar to the x-sheet, but more interactive. You can move your key frames around and adjust your timing easily, and you can also have your recorded audio for animation on another layer and move frame by frame on it.
Although a handful of them still have the classic x-sheet built into them along with the timeline (like ToonBoom and Autodesk Maya), as it provides a clearer and simpler overview.
Nonetheless, be it 2D animation or 3D animation, it is always better to have your animation planned out on paper first, with all the key poses and the timing and spacing worked out. Hence, having a physical x-sheet always comes in handy.