You are not alone in thinking that “the foot must never sickle” in a yoga pose, but the term “sickle” has a negative connotation, just from its sound. It sounds sick! So, don’t sickle. More properly it is called supination: so, is supination bad? Should we, must we never supinate? Well, look at this version of Butterfly (Baddhakonasana)
—notice how the feet are deliberately sickled/supinated. Notice this advanced pose called Kandasasana
by Iyengar: his feet are sickled. Why is okay to sickle the foot in these poses but not in others?
The answer is aesthetics. It just looks bad. And, because it looks bad, don’t ever do it. If you take a functional approach to yoga, and acknowledge the reality of human variation, you will find that what a pose looks like is irrelevant. What is important is what the pose feels like. (See this article
for more on this topic.) Does it hurt your foot, ankle or knees when you supinate your foot in Shoelace? If not, why worry about it? The foot is made to supinate: if you never allow that movement, you may eventually lose the ability to do that movement. If supinating the foot in that pose does cause problems, then, for you, it is not a good idea, but this doesn’t mean it is bad for everyone!
In my recent book, Your Body, Your Yoga
I talk about this is more depth (see pages 243-245.) I hope this helps.