So far as I see, no CoinJoin happened to exchanges. Most of the exchanges like to use the hot-cold-deposit wallet model. Because it's safe and reliable. The hot and cold wallets have obvious features. The cold wallets have a lot of Bitcoin so they are always shown in the rich list (https://chain.info/richlist). And the cold wallet only has one-to-one transactions with the hot wallet, which has a huge number of transactions.

I manually checked the gathered hot and cold wallets of every exchange. They all look well.

But there is also an exception we found later. Some exchanges like to use a changing addresses as the hot wallet. The hot wallet changes its address every time after it generates a transaction.