What replaced the ContentFormControllerSubscriber class?
It was renamed once or twice, but https://www.drupal.org/node/2613034 moved the actual code from an event subscriber to a route enhancer. See \Drupal\Core\Routing\Enhancer\FormRouteEnhancer for the current approach.

Good event subscribers will document how they determined what priority to use, it could have been random or very carefully picked.

See CTools' \Drupal\ctools\EventSubscriber\WizardControllerSubscriber which still uses 29.