It's mouse-right click but on the keyboard (context menu). I believe it's an accessibility thing now that I think about it, which might be why MS is saying "maybe" and considering other keys to replace instead - though that's speculation on my part.
Also, unless I misunderstood this, this is only being mandated for hardware that ships with Windows on it (Eg, surface or desktops that come with windows installed), so this is only being mandated for peripherals that their software is going to run on (or on devices those peripherals come with and connect to).
Will Dell be lazy and ship the copilot key on their ubuntu laptops? I'd be damned shocked if they didn't, but that's just them cutting corners/oversight if I understand this correctly? For example, System76 can completely disregard this if they so choose. I'm sure Apple will.