Disc brakes started to become popular on US cars in the early 70's, carburetors hung out until mid late 80's so a carb and disc combo was possible.

I think the whole SBC failure thing is a bit overblown. Any part on a regular braking system can fail at any time causing loss if assist.

A fan belt can come off and power steering goes away.( Some cars have direct electric assist so that problem is minimized. )

A vacuum pump ( on a diesel ) can fail and power brake assist will go away, yet I don't see a wholesale panic by these car owners.