In the latest versions macCatalyst seems to be done as MacOSX, from my testing with building a project with Xcode (adding to Kiryl's answer)

In reality you can still run stuff on a simulator even if you don't have the correct value, it matters mostly for app store validation, so if you use 3rd party build tools and it's hard for you to differentiate between device / simulator targets for whatever reason (maybe you use a fixed Info.Plist template) you can just hardcode iPhoneOS XROS etc.