FWIW my former
2013 Lexus RX450h (hybrid) produced a sound for this purpose when it was running on battery only at low speed. It could not be turned off via user control, was really only noticeable in the cabin if the windows were down and I was backing-up or pulling into a spot on battery only. The space-ship like sound was not hyped in the original marketing Lexus RXh marketing materials. I had one of the first 2013 RXh's delivered in the US, and it was news on the Lexus forum once I found the sound and we connected the dots, as it was to my dealership when I showed it to them after delivery. IIRC, Toyota put the sound in Prius, as well as some other mfgrs in their vehicles back in 2011/12 because of an in-progress NHTSA ruling that would have mandated it as early as 2014.
More information on this can be found in
Electric vehicle warning sounds - Wikipedia including various country regulations, what makes and models already have it, and reference to the delayed NHTSA regulation voting that would in theory make this mandatory 3 years later if/when confirmed.
Me? In my research before ordering my MS, I found it odd Tesla didn't implement it at the outset given so many others had before and the probability a safety regulation like this would be passed one day. Since I lived with this little "oddity" for years in my former Lexus daily driver, didn't find it annoying, and even with some questioning if the sound prevents accidents or saves lives, I'm all for having it mandated if it helps prevents just one accident or death somewhere. See the above link or the NTHSA site for stats where NTHSA suggests 2,800 pedestrian (distracted, sight-impaired, etc) and cyclist injuries could be prevented during the life of each electric and hybrid vehicle MY if a low-speed warning sound were in place.