I have seen several steel pipe overflows fail after many years, and have replaced a few for customers over the years, I just havent quite figured out why they still use them when PVC is so much easier and lasts absolutely indefinitely if installed properly. I also do know that some old timers are set in their ways and were taught back in the day that that was the way to go, so in their mind I guess, its still the way to go.
That being said, this fellow wanting to put an 18" overflow in a 2A water shed is beyond crazy, for that amount of money you can put in a 6", max 8" over flow and add an additional 2' of free board in a 1/3A pond. like you said, its been overflowing thru a 2' earthen ditch for 20 yrs without washing completely out so the water velocity and volume cant be that enormous. smh
I got a dollar says he had access to some 18" steel pipe that he was going to use up.

All the really good ideas I've ever had came to me while I was milking a cow.