Do Deathknells trigger when saved by Zhonya's Hourglass or other replacement effects?

No, Deathknell triggers do not activate if the unit is saved by Zhonya's Hourglass or another replacement effect.

According to Rule 808.1.d.1: "If the Permanent with the effect is not sent to the Trash, for example because its 'killed' event was replaced with a recall, the Deathknell will not occur."

Why this happens:

• Replacement Requirement: A Deathknell effect is defined as "When I die..." (Rule 808.1.c). For a unit to "die" in Riftbound, it must be "Killed and sent to the Trash" (Rule 808.1.d).
• Intervention: Zhonya's Hourglass is a replacement effect (Rule 369.1). It replaces the "Kill" event with a "recall to base" event (Rule 370). Because the "Kill" event is replaced and the unit is moved to the base instead of the trash, the condition required for the Deathknell to trigger is never met.

As specified in the RiftJudge FAQ #2596: "Since Zhonyas prevents the unit from dying by replacing the death with a recall, the unit does not die and therefore Deathknell does not activate."