In ~5 years on this forum I've never seen anyone mention Option 420 Privacy Glass. Doing a Google search, it appears that earlier X3s and/or X5s offered a Privacy Glass option. A Google search for "30 Jahre M3" turned up some BMW press photos in which the rear windows appear that they may be darker than the front windows, like yours. The BMW NA press release for this model makes no mention of "privacy glass":
https://www.press.bmwgroup.com/usa/a...language=en_US