Heimerdinger - Academy Instructor. For the questions, assume my Heimerdinger is at a battlefield causing the replacement option to be active. Carrion Dredger - [DEATHKNELL] Play a 1 Might Bird unit token with [DEFLECT] to your base. (When I die, get the effect. Opponents must pay 1 Rune to choose a [DEFLECT] unit with a spell or ability.) Does my mech token come in with Deflect?

No — the Mech comes in as a plain 3 Might Mech with no Deflect.

Deflect isn't a modification being applied to the play event; it's baked into the Bird token itself (Rule 187.7 — a Bird token is 1 Might with the Deflect keyword). Carrion Dredger's "1 Might Bird token with Deflect" is just the standard Bird. Heimerdinger replaces the whole token with a defined one: "a 3 might Mech unit token," which per Rule 187.4 is 3 Might and the Mech tag, no keywords. Since the Bird never gets played, nothing carries its text across.

Rule 375 inheritance does cover real modifications — "play a ready token," or a linked "then give it Temporary" — those do follow onto the Mech. But Deflect here is a characteristic of the replaced object, not a modification of the event, so it's lost.

Practical note: the swap is optional per token, and the Mech goes to your Base (the same location the Bird would have gone to), so if you want the Deflect body you can just take the Bird.