Would have been interesting to include an (obviously blacked out) xDrive badge on the left-hand side of the trunklid similar to the M Performance models:
Not that there is necessarily anything to brag about, but there isn't anything to be ashamed of either, quite the opposite, seeing the praise M xDrive has been getting on the other models. Yes, 2WD/4WD selectable M xDrive differs technically from classic xDrive... but it's the 4WD part that matters most, and then BMW itself doesn't shy away from using the xDrive denomination for technically diverse solutions (e.g. current M135i xDrive vs former M140i xDrive).
A missed opportunity of showcasing BMW's 4WD system name on a top-of-the-line model, in light of strong competition from Audi's Quattro on the road and especially in the average buyer's subconscious, which will spontaneously and consistently name the latter as the absolute benchmark.
Maybe there will be an xDrive badge with a free delete option. Or maybe BMW thinks these buyers will typically prefer flying under the radar and only showing their guns indirectly at a green traffic light or on a grimy winding road. Then again Audi's RS models don't wear a Quattro badge on the trunk (but will wear it elsewhere, e.g. in the lower grille), and the latest M5 didn't get it either. Or perhaps BMW believes these cars being seen drifting and doing donuts in 2WD mode (< 1% of the time) would do a disservice to the xDrive brand image...
The most plausible reason is BMW simply wants to set its unique M xDrive system apart from its more pedestrian xDrive versions but doesn't want to create a specific M xDrive brand and badge (unlike Merc's 4Matic+ vs regular 4Matic), therefore intentionally excludes xDrive badging from its hardcore M models; the customer base will know it's available anyway. Even though the trade-off is a missed opportunity of high-end publicity to the average Joes that will be gazing at the vehicles in the street.