Left heel off the Ground
 

I personally never lift my left foot off the ground at the top of the backswing, but I noticed that many great players, great ball strikers such as Sam Snead, Ben Hogan, Jack Nicklaus, Mike Austin…just name a few. I also noticed that in the picture of 9-1-6, it seems to me the left heel was off the ground too.

I put that in my swing yesterday night at range and felt like that was easier for me to keep my head stationary and still got the 2nd axis tilt at impact.

Tiger and many don’t lift the left heel off the ground, is that the body flexibility issue or any advantage or disadvantage of pivot (Zone #1)

Let it happen / Not happen!!
 

My view is that is your left heel naturally lifts that's absolutely fine and if it does not that's fine too.

But if you purposely lift your left heel against your natural tendencies or visa versa can potentially be very bad for your golf swing.

'most' people are not flexible enough to turn fully without letting the left foot come up IMO.

One of the big advantages to "completing the backswing" is that it gives more margin for error - far easier to have the correct downswing delivery path, from the inside.

A very helpful image I have posted in the past to see where you may be is to imagine you "hit the ball with the back of the left shoulder". Odds are good that the majority of folks can't get anywhere near that without letting the left foot come up. Singh is a great example, Austin too.

Correct knee/hip motion makes this far easier. The foot coming up is a result, not a cause. Just get that left shoulder behind the ball and let the body do what it needs to to get it there, under a stable, balanced, center.