World Tiers are now Difficulties?

Returning player here, sorry for the basic question but it looks like a lot has changed since I left. As I recall, some content like helltides, nightmare dungeons etc were only available on world tiers 3 and up, but now with this new difficulty system, is there any context that’s similarly locked out by difficulty? For example, are the 250 cinder chests in helltide only available on a certain difficulty?