Right now, \Drupal\Core\Render\Element\FormElement::processAutocomplete() allows one to pass a route name and route parameters to the URL generator that will build the autocomplete backend URL, but it does not support URL options (query string parameters and such).
This can be quite limiting in cases like the one I just encountered, where I want to alter the autocompleting entity reference query but only if a certain condition is true on the client side (signaled by the presence of a query string parameter).
I think it would make a nice addition to core for autocompleting form elements to support URL options :)
Needs work
10.1 ✨
The change is currently missing an automated test that fails when run with the original code, and succeeds when the bug has been fixed.
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For the tests requested in #2