Does Unyielding Spirit prevent death from Imperial Decree?

No, Unyielding Spirit does not prevent death from Imperial Decree.

Imperial Decree creates a triggered ability that kills a unit when that unit takes damage. Unyielding Spirit prevents damage from spells and abilities — but the kill from Imperial Decree is a separate instruction (a Kill effect, not a damage effect) and is not prevented.

Even if Unyielding Spirit blocks the damage itself, the Imperial Decree trigger still fires and kills the unit. The "Kill" action is mechanically distinct from "Damage" in Riftbound, so damage prevention effects don't interfere with it.