Well, that sounds like it's discontinued, all right. One way that can happen is when one of the optical glasses used in it becomes unavailable. That happened to the 28/35/50 Tri-Elmar.
Edit: Actually I didn't realize there was a 75 Noctilux. I'm surprised: the 75mm Summilux was already pushing most M rangefinders to the limit for focusing. I think 75mm f/1.0 might be a bridge too far, no?