The zombie mode specifically has fan-built browser versions. They are not licensed and they are not the full game.
Zombies is the most requested slice of Call of Duty for browser play, because the mode is self-contained and people want to play a few rounds without a 200GB install.
What circulates are fan projects that recreate or partially emulate maps, plus browser builds of older engines that need data files pulled from a copy of the game. None of it is licensed by Activision.
As fan projects, they exist in the same grey zone as everything else on those lists. They come and go, and the file requirements make them fiddly even when they are up.