As far as I'm aware, it's not available on EventKit. Would you let me know where you've got the data? I'd love to support it if it's available. Thanks!
Sorry, no, you're right, I was reading a Google Gemini summary that suggested the priority property could be used for flagged status, but clicking through to Apple's documentation makes clear that it's it's an int for priority
Definitely displayed as a property in Reminders Suite in Script Editor but I guess that's not useful