I don't think it has to do with fuel efficiency. It's just easier for the ECU to clamp down on one break caliper using the existing ABS sensor, versus having an actual mechanical limited slipping mechanism.

It gets a bit confusing with the Subraus, but it seems like they started moving away from mechanical stuff around 2008?

https://www.clubwrx.net/threads/does-the-wrx-have-a-limited-slip-differential.134360164/

I had an Audi that was all wheel drive, but it had traction control. It felt different on really hard turns and you'd see the traction control light turn on.