Back in June, 'kill trading' was enabled for public Call of Duty: Infinite Warfare playlists. Prior to that change, if two players fired at the same time, and both were on target, the player with the better connection would get the kill. That's because the low ping player's data would reach the host first, meaning that even if the other player fired simultaneously, the host thinks they're dead by the time their lagging packets reach it. And dead people can't shoot guns, so one player lives, and the other dies.
With kill trading enabled, both players die. It changes the texture of duels dramatically. The host ignores that the lagging player was killed before their input was received, because they did fire before they were dead, at least from their perspective. Enabling kill trading in public matches was controversial (opens in new tab)—as every change to CoD multiplayer is—but as of today, that decision has been reversed.
"In June of this year, we enabled Kill Trading in public matches," reads today's patch notes (opens in new tab). "Doing so gave us the ability to test this feature on a large scale. Since then, we’ve heard a lot of player feedback, and have seen a few unforeseen effects on the Scorestreak system and Hardcore modes, so we’ve decided to remove the feature from public playlists today, Friday, July 14th."
Kill trading is still an option for custom matches. The patch also includes a few general fixes, which you can read about in the full patch notes on Steam (opens in new tab).