Karador, Ghost Chieftain + Slick Imitator + Narset's Reversal + Nexus of Fate

How it works

Cast Nexus of Fate by paying its mana cost.
Holding priority, cast Narset's Reversal by paying , targeting Nexus of Fate.
Holding priority, activate Slick Imitator by paying and sacrificing it, copying Narset's Reversal, with the copy targeting the original Narset's Reversal.
Resolve the copy of Narset's Reversal, copying the original Narset's Reversal targeting Nexus of Fate, and returning the original Narset's Reversal to your hand.
Resolve Nexus of Fate, causing you to take an extra turn after this one.
Cast Slick Imitator from your graveyard by paying .
Repeat each turn.

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

1–2 · Exhibition & Core

Spellbook classifies this as Ruthless — not for these brackets.

3 · Upgraded

Spellbook classifies this as Ruthless — not for these brackets.

4–5 · Optimized & cEDH

All combos are allowed.