Liquid is not a real sidechain because it’s federated. Even Adam Back and Sergio Lerner (inventor of the Rootstock/RSK sidechain) will admit this.
Real sidechains work peer-2-peer.
No gatekeeper. No middle men. Are merged mined with the parent chain. Fees go to those who provide the proof of work for executing the sidechain transactions not unknown federation members who can collude to steal your funds.