Have there ever been turn-and-slip indicators that were *not* electric? Every one I've ever seen was electric.
WRENCH1 is right. Well into the 1990's, I flew for that GOM operator (PHI) that had vacuum artificial horizons and directional gyros in their 206B's. My knee-jerk first reaction was that it was a really bad idea and couldn't understand why they did that. Then, about that time, I looked at the cost of replacing the vacuum instruments in my Cessna 150 with electric ones. Holy cow! I just stayed with the old vacuum gauges. And PHI had, what, a couple of hundred 206B's at one time...206B's that *NEVER* flew IFR? Yeah, I'd put vacuum gauges in them too if I were the DOM. In fact, I still would.