As of this change we have changed the behaviour as described by @mcwumbly above, and regular users (including the flaggers themselves) will no longer be able to expand the ignored content. This is the new baseline behaviour, and doesn’t require any additional setup.
We didn’t include the ability to configure whether TL4 users should be able to as part of this change to keep the scope manageable. By default TL4 users can still expand the hidden content.