Break-in is for camshafts only. That is, running at a set speed for a set amount of time, and then only for new camshafts and lifters with a flat bottom (flat tappet).
For a real radical cam for a high performance engine with flat tappet lifters, it is a must, and even then not a guarantee it will live. The mild stock replacement cams can get by with less. All this is due to oil formulas changing back in 2007(ish). Used cams with matching lifters, in the same location can be run as is, its still recommended to add some zinc additive for a while.
Normal driving back in the day was enough to break in piston rings and cylinder ways since most cars and trucks had manual transmissions which by their nature, run the engine up and down with reasonable loads.
Todays cars, even those 30 years ago, have much better materials in them with better oiling systems and better oils, can be driven right off the assembly line and live for what seems like forever. In about 1986, everything switched to roller cams which eliminate all the concern about camshaft wear and break-ins.
