There isn't enough room available in current smartphone camera modules, with the other parts they contain, to add a movable ND filter and the mechanisms to move it, though modules could be made a little bigger to accommodate one, but then the camera plateau would be somewhat thicker.
The experimental OnePlus Concept One from 2020, which was a prototype never commercially released (but demonstrated to some people at CES), contained a layer of electrochromic glass over the camera array, so that when a voltage was applied to this material, it altered its opacity. In the camera’s Pro Mode, the phone could apply a voltage to turn the glass into a solid-state 3-stop ND filter. OnePlus didn't move forward with it, even though they said it didn't add much more expense or thickness, though it might have added a bit of its own slight color cast, and only 3 stops wasn't as much as OnePlus might have wanted to implement (achieving more stops was a greater challenge).