You've provided a succinct and accurate description of sidechains in the context of blockchains, emphasizing their role as separate blockchains that communicate with the main blockchain. The notion of allowing the movement of data or assets between the sidechain and the main blockchain is well-captured.

Your mention of "Steem Smart Contracts" (SSC) introduces a practical example, showcasing how Steemit has experimented with an idea akin to sidechains. The insight into SSC enabling developers to implement and run smart contracts on the Steem blockchain adds depth to the discussion. It's fascinating to see how SSC contributes to the scalability and flexibility of the Steem ecosystem by managing specific tasks without disrupting the core functions of the main Steem blockchain.

Your contribution provides a clear and valuable perspective on how these concepts are implemented and experimented with in the Steemit context. Well articulated!
You can also check out my point of view to the challenge here.