From a legal perspective, the last is pretty close to the first two. If you sign a contract saying those things a court will throw it out. End of story. This is how contracts work in the US too.
Same as how in California non-compete clauses are illegal.
a) There is no such thing as common law in non-UK Europe.
b) Common law was perfectly fine with slavery until it was outlawed by statute.
c) Even in the American system, common law is just one more source of law, alongside statute. Common law prohibits "unconscionable" contracts, but that doesn't mean statute law is prohibited from prohibiting other kinds of contracts (which it does all the time). Hence the boilerplate "void where prohibited" language in all kinds of contracts.
Same as how in California non-compete clauses are illegal.