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Is this still a thing in 8th?

Since here's no such thing as "emplaced weapons" any more, no.
Generally, the fortifications I've seen so far all have a rule which forces them to fire at the nearest target unless a unit is embarked.
That's not to say that there can't be a fortification that uses the occupnts' BS for its shots, but I haven't seen any yet; the closest so far is the T'au Tidewall Droneport which allows its own Drone unit to use the occupying unit's BS but those drones detach and become a separate unit when this happens so it's still not quite there….