i meant on the body of the mirrorless, with the A6000 and the A7RII(if i'm not wrong), there's a focus assist. The A7RII went for a service.
On you mean the lamp on the camera itself? I actually have mine switched off.
This is different from the AF assist beam on the speedlight though.....that system is firing infrared beams at the AF area of the camera to create contrast around the AF area in low light. This is more necessary for a pure PDAF system because of the WYSIWYG nature of pure PDAF.
The contrast detect AF in the likes of the a7rII is using the ISO boosted image on the sensor to focus. The effectiveness of the system is tied to the sensitivity of the sensor itself...this is how the a7s(II) is able to achieve an AF sensitivity of -4EV. The a7s as a whole basically has night vision.
I do wish the autofocus assist beam did work for the a7 cameras though...only because it would probably significantly help low light AF with adapted lenses.