You didn't miss any step. Something must has changed.
I just tested on my end and got the same result as yours. I made Bing search work following the same article several months ago. But this didn't work recently.
This browser is no longer supported.
Upgrade to Microsoft Edge to take advantage of the latest features, security updates, and technical support.
I am unable to add Bing as a result set for search
I have setup the result source as
http://www.bing.com/search?q={searchTerms}&format=rss&Market=en-u
and added it to a query Rule
but nothing gets returned
When i test that query it returns an error
SEARCH RESULT PREVIEW
Sorry, something went wrong.
HIDE DETAILS
Internal Server Error
Correlation ID: c0ff6b9f-0002-0000-45f2-fd8eb5f6177f
I have followed the steps in....
https://social.technet.microsoft.com/wiki/contents/articles/36682.integrate-bing-search-in-sharepoint-online.aspx
Without success.
Has something changed and it is no longer the way to get federated search results, or have i missed a step?
You didn't miss any step. Something must has changed.
I just tested on my end and got the same result as yours. I made Bing search work following the same article several months ago. But this didn't work recently.