What exactly do you mean by passthrough mode? Passthrough is always on. You asked about stable tracking. Did you mean something other than ARKit?

From every comment made in WWDC videos, it's pretty clear that gaze tracking is seen by Apple as a fundamental privacy issues. I wouldn't hold my breath for them to open access to the raw gaze data.

The example is from an earlier WWDC (maybe 2021?). Looks like the APIs haven't changed that much and got a few updates.