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Could you add this to your browser console (F12) and hit return:

const LegacyArray = window.moduleBroker.lookup(
    "discourse/lib/legacy-array-like-object"
  ).default;

  const originalWarning = LegacyArray.warnArrayDeprecation;

  LegacyArray.warnArrayDeprecation = function (instance, property) {
    console.trace("Deprecated array access", instance.__type, property);
    return originalWarning.call(this, instance, property);
  };

then, without refreshing, navigate to admin and see if you can cause the notice to appear.

now copy the message you should see in the console here.