What if they just started shadow banning African Americans? Would that be illegal? This is a serious qeustion. If we have no idea on who they are shadow banning how can we tell if they are not being discriminatory?

The practice of excluding by race is not actually illegal.

Think about fraternities. Think about ethnic centers.

People have an expectation that because it's immoral in the obvious case, it's illegal.

It's actually not. Nothing stops anyone from making a group for, say, Chinese American newspaper reporters, or Lebanese car mechanics.

What's actually illegal is turning people down for jobs or housing over race.

The law would only be interested if the website was the only way to get to protected topics like a job board

It's also illegal in the U.S. to turn away customers because of their race. That's more relevant than private club membership.