What happens when you play Temporal Breach on a unit that is located on Rockfall Path?

The unit is banished permanently and does not return.

Temporal Breach's resolution has two steps: "Banish a unit" (executes — the unit goes to Banishment) followed by "its owner plays it to the same location." Since the same location is Rockfall Path, and Rockfall Path's passive ability says "Units can't be played here," the play-back instruction cannot be followed and is ignored (Rule 359.3.e.6). This is consistent with the established ruling (FAQ #6500, #9325): when a spell or ability instructs you to play a unit to Rockfall Path, that instruction simply doesn't execute.

If the unit was a token, it additionally ceases to exist upon entering Banishment (Rule 183.1), per FAQ #11668. If it was a card, it remains in your Banishment zone with no way to return from this effect.