Stumbling upon this too, there seems to be no way to check whether Reminder is flagged or not (Reminder - Scriptable Docs).
Investigating more, I’ve also found that EKReminder in Apple EventKit is also missing this property (see EKReminder | Apple Developer Documentation), though it is available in Shortcuts (e.g. Get details of Reminder).
Would be great if someone can confirm if my understanding is correct, but the way I see it -Scriptable can only expose properties that are available in EventKit. And even if someone would want to write an app completely in Swift they would still not be able to check whether reminder is flagged or not, since this property is just not part of public API.
Also similar to Adding tags to a Reminder - Scriptable - Automators Talk