Does Allay stack Deflect?

Yes, Deflect from Allay, Eager Admirer stacks with existing Deflect on your other units.

Per Rule 809.2, if a unit has Deflect and is granted Deflect by an additional source, the Deflect Values are summed. So a unit with Deflect 1 (e.g., Vex, Cheerless) that's at the same battlefield as Allay, who grants an additional Deflect 1, ends up with Deflect 2 — meaning opponents must pay 2 Power to target it.

This also works with two copies of Allay themselves: each Allay has its own printed Deflect 1 plus the Deflect 1 granted by the other, so both have Deflect 2.