Quantum computers could eventually become useful for attacks against Bitcoin wallets. But they would need the complete public key associated with an address. In most cases, when you spend coins from a specific address, this addresses' public key is published on chain.

This means: as long as an address is never re-used, the public key remains secret until the funds are finally spent (and the change amount is transferred to another address). See also this related thread.

It seems that in total 25% of all Bitcoins reside on addresses which are vulnerable to such quantum attacks, because their public key has already been published. A part is on old outputs like P2MS or P2PK where the public key was already published when the funds were mined or received. But most of these coins are vulnerable only because the addresses were re-used, and their owners don't know about the problem and/or are lazy and/or are thinking Bitcoin works like Ethereum and your address is your wallet (which is not the case).

The idea is thus: Wallet software programmers could contribute to reduce the amount of these "vulnerable coins" if they show a warning when such vulnerable coins are detected.

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.

As an arguments against this kind of warning, I can imagine some people don't like to be warned all the time because they think the quantum threat is far away or they can manage it. This is of course legit. Thus my suggestion would be to make that warning optional, but enabled by default if you first use the wallet. IMO both types of warnings should be offered.

Some wallets may already offer this, but I haven't seen it still "in the wild" as far as I remember.

What do you think? If there's no real contra argument then I may fill a feature request at least for the wallet I'm mostly using (Electrum) and propose such a warning. I don't like intrusive proposals like Jameson Lopp's panic BIP getting traction where he would force all Bitcoin users to pay tremendous fees to transfer the coins to post-quantum addresses.