I first noticed the convex passenger side mirror on one of my father's cars that he bought when I was a kid. Maybe a '78 Pontiac? I'm not sure.
According to
this post, the standard took effect in 1971, although there must have been a grace period, because my 1974 Porsche doesn't have a passenger side mirror. And I seem to recall throughout the '70s that passenger side mirrors were options, usually included as part of an appearance package on lower-spec cars, whereas high-cost cars like MBs and Cadillacs had them standard. Also, my 1980 911 has a flat mirror on the passenger side with no warning message.
I don't think objects looking farther than they are isn't really an issue because your brain is able to quickly process the whole image and compensate. Kind of like when you watch TV, your brain doesn't interpret the people as being only a few inches high. If anything, I have to remind myself when driving the '80 911 that the mirror isn't showing me a wide angle. As a result, I find that mirror much less useful than a convex mirror.