Answer
Two confirmed approaches: query the SingletonUser (or CurrentPerson) EntitySet on the UserSettings API, or call a projection API that exposes an Identity-style variable directly — either way, treat this as a lookup through a real API rather than a magic built-in variable, and be aware the value doesn't always populate reliably in every workflow shape, so verify it in your specific flow rather than assuming it. See work with variables, scripts, and expressions.