It's because they can host the images in S3 buckets, so that their status page goes down when S3 is down.

/s

But, this really happened in 2017.

if the image tag is drawn dynamically then it is actually probably less bandwidth than the unicode character since the image can be cached at multiple locations including the browser.

Less bandwidth than the 1-4 bytes of a codepoint in UTF-8? How do you figure?

Because if it is cached, then it is 0 bytes transferred. First request could be probably a few hundred bytes, but never needed again. And once it is at a CDN, there is never another request to the server.

.... no way.

Cache freshness checks involve a lot of headers, which take up way more bandwidth than one unicode character.

if you care about cache freshness - you could set the cache header to 10 years and it never be an issue again.