Mercedes-Benz does not publish a general position on length-adding seat belt extenders.
Mercedes' own official term "seat belt extender" refers to a different device entirely: a motorized arm, found in some Mercedes vehicles, that presents the seat belt to an occupant and retracts it afterward.
OEM seat belt extender available: ❌ No
General seat belt extender position: ⚠️ Not published
Mercedes' "Seat Belt Extender" Is Not a Length-Adding Device
Current Mercedes-Benz owner's manuals — including the S-Class Sedan and Mercedes-Maybach S-Class — describe a "seat belt extender" that extends and retracts a motorized arm to present the seat belt to an occupant. This feature does not add length to the seat belt.
A Note on a Discontinued Listing
Mercedes' official parts site also contains discontinued listings for a "Driver's Extended Length Seat Belt" and "Passenger's Extended Length Seat Belt," for the first-generation M-Class (ML-Class), sold from 1998 to 2005. Both are marked discontinued. The available listing information does not establish that these were detachable seat belt extenders of the type discussed in this article, and they do not represent a current Mercedes offering.
Aftermarket Options
If you determine that an aftermarket extender is the best option for your needs, ensure you select a seat belt extender by the exact make, model, year, seating position, and country of the vehicle.
Sources
Mercedes-Benz Owner's Manual: Extending/Retracting the Seat Belt Extender (S-Class Sedan)