The hook_form_workflow_transition_form_alter documented in workflow.api.php is no longer available.
It seems like this is a regression caused by this commit:
https://git.drupalcode.org/project/workflow/-/commit/69e651a039b9cb406e8...
The WorkflowTransitionForm::createInstance method usage was replaced by WorkflowTransitionForm::retrieveFormElement, which does not call the form builder anymore (which was responsible for calling alter hooks).
Implement a hook_form_workflow_transition_form_alter, observe it's not being called.
Mimic the \Drupal\Core\Form\FormBuilder::prepareForm(), call the alter hooks the same way it does.
Review & testing.
None.
Fix of the API regression.
None.
Active
2.1
Code
Note: after a deeper look, it seems like the regression only applies to the field widget. The standalone transition form shouldn't be affected.