Also it should be noted that Imgur images can still be displayed by using the native Markdown markup for images:
BBcode also works:
[img]https://i.imgur.com/6IXPzY7.png[/img]
But this is still a majorly complicated workaround for non-technical users, as there is logically no need for a post editor UI button to “Add an image” thanks to the beautifully simple normal Discourse method of just pasting the URL on a separate line.
So I wonder if an adjustment could be considered for the Discourse code to make it generate the necessary <img> tag in the baked post when the Onebox attempt for an image link fails with a 429 error, and make the post preview reflect that as well.