https://linuxtracker.org/ http://www.publicdomaintorrents.info/ https://ocremix.org/torrents

How would you keep the definition of legality without a centralizing authority?

A tracker is a centralized authority.

But legality doesn't have a central authority. What is illegal in one jurisdiction is ok in another

Just track things that are legal everywhere or in most jurisdictions then.

I don't see how that would be enforceable. Policy perhaps, but it would be impossible to absolutely prevent it from being used for that purpose IMO.