When making your back swing, much has been written regarding the movement in the left foot. In my humble opinion if you are flexible enough to make a full back swing with the left foot planted then do so!

Keeping the left foot planted throughout the swing allows most golfers to create some good resistance and balance, giving a great feel of coiling the upper body over a stable lower body during the back swing and a solid left side to swing into during the down swing.

I only encourage a little lifting of the foot if the client is struggling to complete the back swing with it planted, it simply allows a little more freedom of motion to some golfers, maybe helping them to complete a full turn. Then I ask them to plant it firmly down on the ground at start the down swing, that way giving them balance and a good weight shift in the down swing.

Let me know if this helps?