Mortuary + Masked Meower + Bolas's Citadel + Cauldron of Essence

How it works

Activate Masked Meower by discarding a card and sacrificing it.
When Masked Meower dies, Mortuary and Cauldron of Essence trigger.
Resolve the Mortuary trigger, putting Masked Meower from your graveyard onto the top of your library.
Resolve the Cauldron of Essence trigger, causing you to gain 1 life and each opponent to lose 1 life.
Resolve the Masked Meower ability.
Cast Masked Meower from the top of your library by paying 1 life.
Repeat.

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

1–2 · Exhibition & Core

Spellbook classifies this as Powerful — not for these brackets.

3 · Upgraded

Allowed in this bracket.

4–5 · Optimized & cEDH

All combos are allowed.