Out of the total circulation supply of around 18 million Bitcoin, around 9661 Bitcoins are stuck in different sidechains, which are not lost but cannot be used either. These bitcoins are stuck in primarily 3 major side chains that include Liquid, Binance Chain and Wrapped Bitcoin (WBTC) sidechains.
Side Chains are just like sister chains to the main network which are built to perform specific tasks. Their primary use is to hold the transaction amount when the user sends the fund to or from the sidechains.
As per the latest data from block explorers, Liquid network contains the smallest amount of Bitcoin stuck in sidechains with 89 BTC, while WBTC contain 571 BTC and Binance Chain has the highest number of Bitcoin stuck with 9001 BTC.