Do banished cards still trigger their Deathknell?

No, banished cards do not trigger their Deathknell ability.

According to Rule 808.1.d, the trigger condition for Deathknell specifically requires the permanent to be Killed and sent to the Trash.

As established in RiftJudge FAQ #11055 and #6615, Banish is a distinct game action from Kill (Rule 427.2.a). Because banishing a unit sends it directly to the Banishment zone rather than the Trash, the requirement for the card to be "Killed and sent to the Trash" is not met. Therefore, the Deathknell ability cannot trigger.