Crossplay: Some people like it, some people don't. It's always nice having more people to play with, but console players will sometimes switch it off to avoid being mauled by the speed and accuracy of mouse and keyboard (or, you know, cheats); likewise, PC players sometimes prefer to avoid the accuracy-boosting aim assist offered by console shooters. But for Xbox and PC players on Call of Duty: Modern Warfare 2, that is literally not an option.
According to multiple complaints on Reddit, there is simply no option to disable Modern Warfare 2 crossplay on Xbox or PC. I can't speak for the Xbox, but I checked the game on PC and it's true: There's no toggle to turn it on or off. What makes the situation really strange, though, is that multiple redditors say the PlayStation 5 version of the game does have a crossplay toggle. Eurogamer verified that the PS5 menu option is present.
It is possible to turn off crossplay on Xbox consoles by disabling it completely through the console's privacy settings, but that's a hassle for anyone who wants crossplay enabled in other games, just not Modern Warfare 2. And of course it does no good for PC players, who are stuck with crossplay enabled.
It's a weird situation. It'd be one thing if Activision made crossplay mandatory for everyone—not necessarily a good thing, but at least understandable. But to make it an option on one console but not the other (and not PC) is a bizarre decision—assuming it's not a bug, of course. Some players on the Modern Warfare 2 subreddit have theorized that the call was actually made by Microsoft because it wants to combine Xbox and PC into a single ecosystem, but I think that's very unlikely: At its core, removing the crossplay toggle is a denial of basic functionality, and that's not going to make anyone happy no matter what the intent.
PS5 network settings, with crossplay toggles at the top:
PC network settings—note the absence of crossplay options:
I've reached out to Activision to ask why crossplay cannot be disabled on PC and Xbox, and will update if I receive a reply.