The heel or flat discussion pertaining to the stride foot plant is a matter of degrees, and even that has to do with the physique of the pitcher , in addition to their physical maturity/coaching.
John Lester is a bulldog of a competitor. He’s big, aggressive, and loaded with power to spare. In picture -1- his approach down the mound seems like he’s walking his stride, with his stride foot’s toe up, about 45 degrees. At the last minute before landing, picture -2-, his stride foot almost flattens out 100%. That’s because he’s about to drive all that mass and bulk physique of his, downward into the ground slight via his stride leg. In our picture -3- we see the short version of his complete pitching cycle. There, his stride leg is accepting all that mass of his downward into the ground, the hips are acting like an axis where his upper body’s mass is driving forward, releasing the ball, and his pivot leg is a balancing all the forward weight. In picture -4- all of this put together shows that pitching (ball) coming right off his pitching shoulder and right down range to his backstop.
Early pitcher development has a lot to do with this topic also. Physical makeup (physique) as well as professional coaching can have a great influence on a pitcher. Randy Johnson in picture -5- was in the very early stages of his career with this picture was taken. Notice the leading heel of his stride foot. As Johnson progressed both in physical strength, body composition, and I’m sure some professional coaching along the line, his stride foot approach was different.
Greg Maddux has a stride foot approach that, at first, appears to be heel first, but it’s not. His stride foot has his toe pointing sideways a bit, thus allowing him to stay closed in the hips as long as possible. At the moment just before landing on his stride foot, his foot points towards his backstop, flat and in contour with the mound. Refer to pictures -7-, and -8-.
By the way, those pitchers in youth baseball tend to use the heel first landing, a lot, mostly do so because of the lack of coaching, strength and coordination issues, in addition to poor surface conditions on the mound. I’ve also seen youth pitchers who are fatigued use the heel landing a lot.
Personally, I’m not persuaded to judge one over the other too much. One method or the other seems to work better for some.