[badsectoracula]

I'm certain multiple mice is (or at least, was) supported on Windows, i remember some of Reflexive Entertainment's games from around mid-2000s having what they called "Mouse Party" which was basically a local multiplayer mode with multiple mice connected to the same PC (their games were casual indie games with simple control schemes that often supported something like that).

In theory multiple mice and keyboards are possible on X too since late 80s/90s, at least assuming the XInput extension is available on the server since the core events do not provide any way to differentiate between devices. But like everything with X, the feature might be there, but it is up to the clients to actually use it and use it properly (which isn't always the case and a lot of "X cannot do <thing>" you hear really means "due to the way we use X, we cannot do <thing>"). And even if the feature is there, if it is a niche case, it may be so buggy that it isn't practically usable (of course this applies to almost every open source project that is older than a couple of years, especially those that are big and have a small number of contributors).

An example of this would be multiple focused windows (one per keyboard) - despite XInput allowing for such a thing, i doubt many window managers and toolkits would be able to handle it (and to be frank, neither does my own toolkit and most likely will never have support for it - it is so niche of a feature, it isn't worth the time and increase in complexity and potential future maintenance).

No idea about macOS.