among them BTSE, Sideswap, now Aqua
Never in my life had I heard about the first 2, Aqua is not an exchange, it's a wallet, still you can see that Liquid's blocks are completely empty most of the time.
we can store L-BTCs with a hardware wallet
You can also store, RBTC, WBTC, renBTC, BTCB, BTC.b, etc with a hardware wallet, what's the advantage to those things?