I am not really sure when logic of keeping the left heel planted came about?
I do believe it has done more harm than good to golfers swing and in particular to golfers bodies.
Proper legwork and footwork is the essence of proper motion.
Staying flat footed and planted inhibits proper legwork and it makes the transition move just that much harder and forced rather than floating and free.
Here is a video of a student lesson showing a drill designed to encourage a smoother transition
where the hands can drop nicely into the slot (like Tony Lema above) without being forced down to prepare for impact.