If Tasty dies outside of a Showdown and it's your only unit, can you still reveal/activate Zhonya's in response to Tasty's Deathknell trigger?

Ruling: No, you cannot reveal Zhonya's after Tasty dies if it was your only unit on the battlefield outside of Showdown. The hidden card is removed during cleanup before players get priority.

Sequence:

• Tasty takes lethal damage and dies
• Cleanup notices Tasty has lethal damage and kills it
• Follow-up cleanup for Tasty leaving the field
• Game notices you no longer control the battlefield and attempts to put Deathknell trigger on the chain
• Another cleanup removes Zhonya's because the battlefield status changed from controlled to uncontrolled
• Players get priority with Deathknell on the chain (but Zhonya's is already gone)

Nuances:

• You must activate Zhonya's before Tasty dies to have it do something (which would likely prevent Tasty from dying)
• In a Showdown, the rule is different: control of the battlefield doesn't change until the end of Showdown, so you CAN play Zhonya's after your unit dies to save the hidden card
• In a Showdown with Tasty specifically, you could play Zhonya's either before or after the Deathknell trigger