There are no absolute rules here, indeed. The “knee not beyond the toes in front stance” principle is a rule of thumb (rule of toe…?) that is usually useful, especially for beginners, but not an absolute necessity. Doing a grand plié can be fine for the knees (and great for the leg muscles) if you do it right. I can refer to my own 8 years of ballet training for that. (No, that's not me in the photo. Neither of them).
What’s more problematic is when the vertical line down from the hip joint (which also marks the greater part of the load created by total body weight) no longer ‘falls’ in the heel or the centre of the foot (up to and including the yong chun point/Kidney 1). The knee itself can be fine beyond the knee in vertical terms as long as the inguinal crease goes
deeper with the stance, no problem.
But if this vertical line goes further forwards, beyond the toes, with the hip itself pushing forwards instead of sinking down, then the knee joint is subject to stresses that otherwise would not occur. Try it, you can feel it
. As said, that may be fine for some people but for others this is where the knee problems start or get worse if this is done habitually.