What replaced the ContentFormControllerSubscriber class? In the past, there was a ContentFormControllerSubscriber class in Drupal, but there is no track of it anymore; the change records don't report anything about that class too.

The reason I am looking for that class replacement is that I noticed the Chaos tools module using a similar class to alter the requests passed to Drupal, and I was wondering if the priority (29) used in the getSubscribedEvents() method of those classes is still sufficient to alter the request before it is too late, and not doing it too early.
I guess the Chaos tools module used that value because it was used from Drupal core too; I am assuming there was a good reason to do so.
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.