When the 351C was introduced Ford informed the press and their own employees that the engine was planned for 5 a year production run. I don't know why it was only going to be produced for 5 years, but since the announcement was made in 1970, I suspect it had nothing to do with tailpipe emissions compliance.
It was emission that finally lead to the engine's demise in the US (351M & 400) however; and probably Australia too (302C & 351C). The engines were cancelled the same year (1982) on both continents. Of course the engine lingered on in Australia at least through 1984, 2 years after its production officially ceased. The notch in the piston dome providing clearance for the canted intake valve made it necessary to situate the ring package lower on the piston. Hydrocarbons collect in the gap between the piston & cylinder wall above the top ring. Since the ring package sits lower in the clearance, more hydrocarbons collected, thus raising the engines hydrocarbon emissions.
If they had modified the Cleveland intake valve geometry just enough to eliminate the notch in the side of the piston, thus allowing the ring package to be raised, the head would have made a nice performance head for the 5.0 HO engine. A 1985 version of the Boss 302. I think it would have made sense from a marketing stand point, and made a few extra horsepower too.