The new update added the ability to turn on things like coordinates, fire spread, and tnt without losing achievements. However, this can not be used on a pre-existing Realm world.

In the Realm menu settings for the world (that has already existed prior to the update), there is simply a button for "Activate Cheats." If you click it, it disables all achievements. Therefore the new feature cannot be applied to a Realm world.

All the new features are hiding in that button.