Does casting Imperial Decree during a showdown result in enemy units still dying to the delayed trigger from Imperial Decree after using replacement effects during cleanup such as Zhonya's Hourglass or Sett, The Boss.

Yes — the replacement effect only buys the unit a single death. Saving it in the cleanup doesn't remove or weaken the Imperial Decree trigger created by the damage.

Damage being dealt is what fires Decree ("When any unit takes damage this turn, kill it"), so that kill trigger is already a pending chain item the moment combat damage is assigned. Cleanups can't finalize or resolve chain items (CR 320), so the lethal-damage death happens first and Zhonya's / The Boss replace that one death (heal, exhaust, recall) — but the pending "kill it" item then finalizes and resolves afterwards and kills the unit anyway. Decree has no location restriction, so being recalled to Base doesn't protect it, and it's the same precedent as Guardian Angel in FAQ #12272.

It can't save twice: Zhonya's kills itself to pay for the save, and The Boss requires both exhausting itself and spending the unit's buff, so neither is available for the Decree kill (and The Boss can't cover a unit that isn't buffed at that point). Also worth noting: a unit whose damage was non-lethal behaves identically — it survives the cleanup and then dies to the trigger.

The one way out is a second, separate save held back — e.g. a face-down Zhonya's is only playable once those pending items have finalized and someone has priority, so flipping it in response to the Decree trigger does save the unit (FAQ #12784).