This is a very good thing designed to eliminate “pinch boots.” They affect horses very differently…some don’t seem to care at all and then sometimes there is a huge change, like Admara at the World Cup Finals last year: https://m.youtube.com/watch?v=q3R4o2pNB0w&feature=youtu.be
the rule is somewhat over inclusive…I have the Thinline ankle boots PNWjumper linked and they certainly don’t pinch when applied correctly. But the turnback Velcro could possibly allow overtightening more easily, so I will happily use something else in the unlikely event I show at FEI level.