After the CR update, can a Hidden card be played in response to a Deathknell trigger during combat?

Ruling: No, you cannot play a Hidden card in response to a Deathknell trigger. By the time the Deathknell trigger is placed on the chain, players have already lost attacker/defender status during the combat special cleanup, meaning no showdown or combat is staged at the location and the Contested status is cleared.

Sequence:

• Combat special cleanup occurs, removing attacker/defender status and Contested status
• Deathknell triggers are placed on the chain (before units die)
• Units (both Deathknell and non-Deathknell) die and go to trash in the following cleanup step

Nuances:

• Before the CR update, it was possible to use Hidden cards in response to Deathknell triggers during combat, but this changed with the clarification that removing Contested status is NOT part of the special cleanup
• Outside of combat, if a Deathknell unit is killed by a spell, the Hidden card is removed from the battlefield in the following cleanup
• Karthus (passive ability) will see Deathknell triggers created and cause them to trigger twice when dying simultaneously, because the Deathknell trigger is placed on the chain before the units die
• Viktor Leader will not trigger when dying simultaneously with other units because his trigger evaluates precisely when units die