It may not be enforced, but the Microsoft ToS states "You agree not to use any false, inaccurate or misleading information when signing up for your Microsoft ac

It may not be enforced, but the Microsoft ToS states "You agree not to use any false, inaccurate or misleading information when signing up for your Microsoft account." (https://www.microsoft.com/en-gb/servicesagreement/ https://www.microsoft.com/en-gb/servicesagreement/), and the signup flow does require a full name.