Update- December 22nd, Feedback Reopens
Apple has reopened feedback applications for iOS and macOS, a reason behind the closure yesterday is still unknown.
Apple Terminal can confirm, as of right now, feedback for iOS 13.3.1 beta 1 is closed. We are currently unaware of the reason.
Apple has also closed beta for macOS Catalina 10.15.3. Currently Apple has not sent any message or email on why feedback is closed, maybe due to the holiday however we are unable to confirm that right now.