A timeline chart displays when a page has ever been "unhealthy". The thumbs up or thumbs down emoji next to the "Health" header represents the most recent check.
What is an "Unhealthy" Page?
By default a page becomes classified as "unhealthy" when a check finds the HTTP response status code to be 400 or above (an error status code). Users can change it to the opposite in page management by selecting "HTTP healthy status code" under the "Unhealthy condition" dropdown in "Health Check Settings".