I cannot now check the early configurator but nobody has commented yet that their early 350+ UK model had rear-wheel steering so that adds to the evidence.
Rear wheel steering is configurable now in UK for more premium trims of 300 but originally when 350 was orderable in UK, journalists like Top Gear either didn't have rear-wheel steering in the cars available to them or even said that rear-wheel steering simply wasn't configurable (and it's not necessarily a limitation of 350 as 350 can be configured in other countries with rear-wheel steering):
Image is snapshot in time but Top Gear may revise the webpage of course if they test drive newer models:
Mini-me EQS that might be comfier still than its big brother. Would be more relaxing if the tech was less overwhelming
www.topgear.com
I am guessing it could still have been ordered at that time but would have been a special order with a special waiting time.