The additional 'parameters' line in this update is added to the beginning of the development.services.yml, as it should be.... but - some older tutorials relating to enabling twig debugging may have led people to have already added parameters: twig.config: debug: true to the end of the file.
Running the updates will add the new parameters at the beginning, resulting in two declarations of parameters, and then twig.debugging obviously ignored.
This will have the effect of twig debugging disappearing. The multiple declaration causes no errors, so I did not notice it at first.
This only will affect anyone coming back to an older than 8.2.4 build and running the updates.
No further action needed - this is just to help anyone searching for the issue as I was.