Script – What is OP_RETURN used for in Bitcoinbase transactions other than obvious spam?

One day I heard from Bitcoin Mechanic that there is a CoinJoin implementation that leaks some of the data needed to determine the price. Is that true? What do custodians like Liquid use OP_RETURN for? Besides various forms of spam, what is OP_RETURN used for in Bitcoinbase transactions? Or, that is, assuming that from tomorrow onwards there will be no OP_RETURN other than the OP_RETURN used for Coinbase transactions. What would break in such a scenario?