How keyword stuffing metric is defined?

Over optimization risk is an “overall” metric.
It just means you have used a lot of terms (from the sidebar) compared to the length of your text.
Usually keyword stuffing risk gets lower when you write more content.

Where can I find Over optimization risk metric for my article in the Editor?

This metric you can find in the Checklist window.

To find keyword stuffing metric, follow these steps:

  1. Go to the Content Optimization tab.
  2. Navigate to the  Optimization balance metric.

New Optimisation Tab