There is no such thing as “the rules of kickboxing”
Sixteen codes are in active use and they disagree about leg kicks, knees, the clinch, throws, head punches and footwear. Almost every argument about the sport is really an argument about which rulebook you mean.
Ask what is legal in kickboxing and the honest answer is a question back: under which code?
Under American full-contact rules nothing may be thrown below the waist. Under low-kick rules the leg is a primary target. Under K-1 rules knees are legal but the clinch is broken within a second. Under Muay Thai it is a scoring phase you may work in indefinitely, and elbows are legal too. Under savate the shin may not strike at all and the shoe is the legal surface. Under sanda you may catch a kick and throw the kicker, and it scores.
These are not variations on a theme. They select for different athletes. Bill Wallace retired undefeated as a champion with one functional kicking leg, which is a career that exists only inside a code where nobody may attack the lead leg. Semmy Schilt won four Grand Prix titles largely on a front kick, which works because K-1 breaks the clinch and removes the standard answer to a much taller opponent. Rick Roufus lost in 1988 to a technique his own rulebook had never required him to defend.
So when someone says a fighter is the best, or that a technique does not work, or that one art beats another, the useful follow-up is always the same. Under which rules?
That question is the organising principle of this entire site. Every technique page lists the codes that permit it and the codes that ban it. Every fighter is placed in the ruleset they actually competed under. The comparator exists so you can put two rulebooks side by side and see exactly where they diverge. It is not pedantry — it is the only way the sport makes sense.