Such a warning could be implemented e.g. in two variants:
1) Warn if you are about to create a transaction which transfers only a part of the funds on an address to another address. This would make the remaining coins vulnerable to quantum attacks.
2) Warn always if coins are detected where the public key was already published on-chain (both those where already a transaction was sent from the same address, and those on P2PK or similar outputs). Offer a button with info about which utxos (coins) are currently vulnerable.
Why do I have a feeling that this is going to spark a lot of controversy and debate because this is going to look like wallet developers are going to become onchain surveillance, watching addresses that are public key exposed and the ones that are not exposed on the Bitcoin network. We are bringing a solution to take away little privacy we enjoyed with Bitcoin.
Unless it's going to become a consensus rule (with optional) for all nodes to have a special pub_expose(if there is any other name we can call public keys exposed) to have all addresses that are public key exposed on the Bitcoin network so that all other nodes can communicate with each other and propagate this addresses to each other but then again, this is going to look like chain analysis.
What do you think?
I'm not sure if some people don't really care about updates or threat on the Bitcoin network, it's just that some people like having things the way they are as long as it doesn't make their private key exposed or threaten.
We can't predict exact time Quantum computers will become fully operational but if today a solution is provided, everyone will adopt it for their own safety and security of their private and public keys.
Which ever way we try to treat this with caution, there is going to be a every high transaction surveillance between wallet addresses whether the coins are spended or not if people decide to send coins from one address to another for the fear of having public keys onchain.