Can you activate a hidden Zhonya's in response to a Deathknell trigger during combat cleanup when your unit dies to combat damage?

Ruling: Under current rules as written (RAW), Deathknell triggers during the special cleanup after combat damage, and you can respond to it with hidden cards like Zhonya's because contested status is not cleared until after the special cleanup completes (step 440.1.b). However, this creates an ambiguous situation that is not fully covered by the rules and will be changed in version 1.2.

Sequence:

• Combat damage is dealt
• Special cleanup occurs (440.1.a)
• Deathknell trigger is added to the chain as a pending item
• Players can respond with hidden cards while battlefield is still contested
• After chain resolves, contested status is cleared (440.1.b)

Nuances:

• The substep called "Combat Cleanup" is not actually a "cleanup" as defined by rule 318, which is the source of confusion
• Control of the battlefield cannot change while it is contested (rule 181.3.b)
• This interaction only occurs when the unit dies to combat damage, not when killed by spells/abilities during the showdown
• For tournament play, defer to the head judge's ruling as the rules have a gap here
• The intended behavior (RAI) is likely that hidden cards should be removed and Deathknell should trigger after the showdown resolves, but this is not what the current rules say