Decoppering agents have been around for a long time, but not in many handloading powders. They're really pretty simple, a slight amount of some soft metal that combines with the copper, making it brittle, whereupon most of the copper's blown out of the bore with the next round.
The oldest decoppering agent is lead. It doesn't even have to be combined with the powder; artillery shells often had a coil or piece of lead foil placed on top of the powder charge, under the projectile. (As you might imagine, it's a PITA to clean copper-fouling out of a cannon barrel.)
But decoppering agents have been in a lot of military small-arms powders for years, mostly due to automatic rifles, and when those same powders are sold to handloaders they work well. Ramshot TAC, for instance, is a Belgian powder originally designed for NATO military rounds, the reason it contains a decoppering agent and the other Ramshot rifle powders don't, even though they're made by the same factory and use the rest of the same basic technology. Some of the older ball powders sold by Hodgdon also have long had decoppering agents.
These days lead pollution is unpopular, of course, so other decoppering agents are used. Bismuth is a good one, for instance.
I tested the decoppering of IMR4166 by shooting my Thompson/Center Icon .223 for 100 rounds without cleaning, while making other tests with the powder. The Icon's barrel doesn't foul a lot, but it does end up with enough copper inside after 100 rounds to show up plainly in a bore-scope, and to turn a solvent-soaked patch a definite blue. After cleaning out the slight amount of powder fouling with some alcohol-soaked patches, there was only a hint of any copper in a few places inside the bore, and a patch soaked in Montana X-Treme Copper Killer showed only a very faint hint of blue.
