Can a Reflection token of Ekko trigger his Deathknell effect?

Yes, a Reflection token that has become a copy of Ekko, Recurrent will trigger its Deathknell effect upon death, but it will fail to ready your runes.

Why this occurs:

1. Ability Inheritance: When a token becomes a copy of a card, it gains that card's abilities. As a valid game object, the Reflection token is capable of triggering its Deathknell ability when it is sent to the trash (Rule 808.1).
2. Triggering vs. Resolution: While the Deathknell ability successfully triggers and enters the chain, the Reflection token ceases to exist as soon as it enters the trash (Rule 183.1).
3. Mandatory Cost: The Deathknell effect of Ekko requires you to "Recycle me" as a cost to ready your runes. Because the Reflection token ceases to exist upon entering the trash, it cannot be recycled. Consequently, the mandatory cost cannot be paid, and the effect fails to resolve.

Note: I could not fully verify this ruling against the retrieved rules.