If the private key to a UTXO is lost, the UTXO remains on the blockchain forever and is effectively unspendable.
I am not sure you fully understand what UTXO's are, but anyway, UTXO's do not have private keys. It is bitcoin addresses that have private keys. Unspent Transaction Output (UTXO) is simply bitcoin you have received in your wallet that you are yet to spend, and whenever you decide to spend some or all of it, it becomes the input in the new transaction.
Is there any way these coins could ever return to circulation? But then the bigger question is, how would we even identify a lost UTXO?
No, coins that are lost are lost for good.
No, there is no way to identify lost coins. There are coins sitting in wallets that haven't moved for a very long time, and some people might think the owners have lost the keys, meanwhile, they are just solid long term hodlers.

A few days back I came across a post here about protecting your Bitcoin while still making it available for your family in the event of your Given Bitcoin’s fixed supply of 21 million coins, could lost coins eventually make the available supply much smaller?
Yes. Bitcoin has a fixed supply, so lost coins reduces that number and makes everyone else's coins worth a little more.
Lost coins only make everyone else's coins worth slightly more. Think of it as a donation to everyone.