The term “sidechain” itself has multiple definitions, Smart BCH will launch as a federated sidechain but intends to shift to a decentralized bridge called “SHA-gate” where miners vote on outgoing transfers in a covenant contract. You can read more about it on the Smart BCH documentation https://docs.smartbch.org/smartbch/sha-gate#the-design-of-sha-gate
What not many people know is Bitcoin also has a EVM compatible sidechain called Rootstock or RSK for short which launched in 2018 already. Rootstock intends to move to drivechain, a decentralized bridge that has similarities to the SHAgate idea. Apparently there’s also the idea of having SPV verification inside the blockchain itself as a sidechain mechanism with Proof-of-work.
Sidechains played an important part in the vision blockstream sold to their investors but they never delivered on their idea of sidechain constructors – a shame because the idea has a lot of potential. Instead they just developed their closed source federated liquid sidechain instead of a noncustodial and decentralized 2 way peg. See this great article ” Is Liquid A “Real” Sidechain? Greg Maxwell Weighs In”