Is it against Xbox Live policy to play on multiple accounts?

On a policy level, does Xbox Live oppose the notion of a single player owning and operating multiple accounts in multiplayer games, especially PVP multiplayer games such as Overwatch, Fortnite, Apex, Halo, Call of Duty, etc.?

Microsoft publish a general purpose Services Agreement which covers all services offered by Microsoft including Xbox Live.

It includes a Code of Conduct section that links out to an Xbox Live specific interpretation called the Xbox Community Standards.

I recommend reading both documents carefully and interpreting them for yourself.

However, I think the most relevant part is in the Xbox Community Standards where there is a section entitled Harmful behavior has no place on Xbox; we are expected not to "use multiple profiles to unfairly manipulate your multiplayer rank".