If I Temporal Breach the target of Blitzcrank, Impassive, does it still have to move?

No — the unit comes back and Blitzcrank's move instruction is ignored, because the breached unit is a brand-new game object and no longer a legal target (Rule 359.3.e.4).

Sequence: Blitzcrank's "when you play me" trigger goes on the chain, creating a Closed State, so the only way to Breach in response is from face-down (Hidden grants the Reaction timing). Breach resolves first, banishing the unit and replaying it to the same location; then Blitzcrank's trigger resolves and its chosen target has changed to and from a non-board zone, so the choice is invalid and "move an enemy unit to here" can't be followed (Rule 359.3.e.7).

The practical gate: the Breach must have been hidden on a previous turn at the battlefield where that enemy unit currently sits, since a Breach played from face-down can only choose game objects at the battlefield it was played from (CR 811.1.d.2). It can't be cast from hand in this window.