<sigh/>

What, exactly, is the abuse case which motivates Uber to disallow sending any message to the driver you just rode with?

Few customers would care if such messages had to fit some simple template: "I may have left my [phone|wallet|groceries|dog|cosmetics|handgun|other] in your car, please check". And it'd all be in Uber's system, so they could notice abusers or just customers with memory issues.