Iridescent Tiger + Cloudstone Curio + Roaming Throne

Decks

How it works

You control and own a nontoken nonartifact creature that can be cast using at most six in any combination of mana.

Cast Iridescent Tiger by paying .
When Iridescent Tiger enters the battlefield, it and Cloudstone Curio trigger twice.
Resolve the Iridescent Tiger triggers, adding .
Resolve the first Cloudstone Curio trigger, returning a nontoken nonartifact creature that can be cast using at most six in any combination of mana from the battlefield to your hand.
Resolve the second Cloudstone Curio trigger, choosing to return nothing to your hand.
Cast the creature returned to your hand by paying its mana cost.
When the creature enters the battlefield, Cloudstone Curio triggers twice, returning Iridescent Tiger from the battlefield to your hand with the first trigger and returning nothing with the second trigger.
Repeat.

In which brackets is this combo allowed, per Spellbook's Exhibition tier?

1–2 · Exhibition & Core

Allowed in this bracket.

3 · Upgraded

Allowed in this bracket.

4–5 · Optimized & cEDH

All combos are allowed.