This is wrong in many ways. Pre-selling something you don't own yet is always a bad idea.
It would oblige exch to keep user funds, an additional unnecessary security risk.
It would no longer be instant exchanges, functionally the same as other CEX, it is not what we users want. Funds somewhere without control, for an indefinite period
To manage the price, a significant up or down of the currency price can happen, so the customer will not receive the amount that was planned at the beginning of the swap process.
I assume that such a list would have to be public because you don't want to send Bitcoin without knowing that there are already 20 open orders pending. A public list on exchanges which prefer anonymity seems like an illogical solution.
Like I said, it would be an option for those who want to use it. Obviously, it shouldn't be the standard workflow. I don't see the problem with trusting the exchange as custodian of my coins for 12/24 hours (if I wanted to), if I already trust it as the custodian for instant exchanges. They still have the coins, only the period of time they are in possession of them changes. Many of us have and still are using mixers, and you probably know that some have options to "pay out" or provide you the private keys with a delay to make tracking even more difficult.
Sure, it would no longer be an instant exchange for users who are fine with that. Price swings could affect the final amounts, but that's already the case today. In the previous few days, we have seen bitcoin jumping between below $61k to $63k sometimes a couple of times a day. For the time it takes for your BTC transaction to confirm on-chain, you can, in theory get $2,000 less per bitcoin due to this volatility.
There is great demand for Monero. That's where this could be useful. There is also great demand for USDT now. Reserves sometimes show several tens of thousands of dollars. Then 10 mins later, it's down to 90 USDT, for example. Why shouldn't I have the option to be placed on a waiting list to prevent such situations if I wanted to? If there are no reserves for the coin I want, the exchange has to send back my deposit. I then have to deposit a second or third time if I am unlucky. That's throwing away money on fees.