That already exists. That’s the men’s category. There are no rules forbidding women from competing in men’s categories at Olympic events.
Women are not excluded from golf tournaments, but the requirements to compete (primarily how far one hits the ball) are vastly different. Thats why both play the same golf course, just from different tee boxes.
If it's called "mixed league" the intention is clear
So maybe I think what they mean by "mixed league" is not a "Maybe Mixed League" but like "Definitely Mixed League" as in mixed participants being a strict requirement somehow?
Mixed league works fine for your company softball team. Not so much for professional sports.