Set default _format query string

This is a good feature request, but out of scope for this module, I think that would be for core.

OK, as I suspected.
Where should I re-raise the issue?

You can create the feature request here: https://www.drupal.org/project/issues/drupal?categories=All →

I'd do a search first to see if the issue already exists.

Arguments for and against, all valid...
https://www.drupal.org/project/drupal/issues/2852587 →
I think I'll let it lay!