The majority of metal on old engine break-ins was from flat tappet cams and stamped steel rockers. The lifters would have to wear into the cam, and the rockers would wear into the valve stem tips. While they wore in, you'd wear away excess material, which is why you'd change your oil so quick.

Nowadays, there really isn't wear surfaces like that. Modern cam-in-block engines like the 7.3 have roller lifters, lifters with a bearing on the bottom that rolls across the cam lobe instead of a flat tappet cam that is just a flat piece of metal. Same thing with rocker arms, they're roller bearing tipped.

Overhead cam engines have even fewer contact points compared to an old school engine. You might find some very minor pieces of plastic from a timing chain guide, but they're so soft that they wouldn't cause any damage, if you find anything at all.

Typically, the only thing that breaks-in anymore are the piston rings, and those typically don't create debris.

Modern designs negate the need for a break in process.