How long have infants been taking vitamin d3 supplements? is this generally a new recommendation? is it definitively safe? the older generations for instance did not do this?

For 2 or more decades that I recall but it is more accepted the further you go from the equator & among populations with less sun exposure. People didn't wear seatbelts before they emerged as a benefit. Some folks will still not wear them, and suffer as a result. It is safe at recommended dosing. Poor vitamin D levels are associated with many afflictions and poor immune function.