Well you won’t be allowed to participate, and could be trespassed if you refuse to follow the rules… so kinda

That's not remotely "by law."

If I could start a league tomorrow where shin-guards were optional, would the police come knocking or not?

If not, there's no law. There's private rules.

The whole question under debate is whether actual laws are necessary for this situation. I fail to see how appealing to voluntary association rules have any bearing on the question.

You wouldn’t be allowed to use public fields without insurance, and no one is going to insure you if your whole thing is “no safety gear”. So you would have to use a private field.

So the practical effect is identical to seatbelts; you can absolutely operate without seatbelts/shin guards, but only in completely private facilities.