I would guess one of the rarest parts that are not only no longer available but, when asked to make one is greeted by "HUH??" from today's manufacturers, is the two-color convertible top available on the 1961 Oldsmobile Starfire. I think this is probably the ONLY convertible ever with a two-color top. If you have not seen one, there is a contrasting color strip on the edge of the top:
https://www.mecum.com/lots/AR0612-129552/1961-oldsmobile-starfire-convertible/Since this was a 1/2 year car and they only made about 8,000 of them, and since you had to specify this option, not many were made. Funny, the major top makers today had no idea what I was talking about when I asked them to make one several years ago when I was trying to buy one of these rare cars. People that own them hardly ever sell them, and many that are around today are missing MANY rare parts for them such as the hubcaps and rear and hood trim pieces as well as the carpets with the inserted metal trim.
So, this is my candidate for another of MANY rare parts, but one that probably COULD be reproduced today by a top manufacturer (Robbins?) that tried!
Cheers!