FWIW I have four different metal cards and have not received this warning with any of them. In the case of the Apple Card, I'd say it's due more to the fact that the card is white than that it's metal.
Hell, some banks even use the customization of debit card appearances as a selling point for extra fees.
And yet, not with a single one of these have I ever seen anything warning about the card becoming damaged/discolored/scratched. Indeed, I personally have multiple AmEx cards, as well as the CSR and CSP, and after years of use (and gasp storing them in a leather wallet) there is very minor discoloration on one of them while the rest still look great.
The entire discussion about the appearance of a damn credit card of all things is absurd. It was absurd when every other card issuer did it, but it's even more absurd now that it seems to have been brought to the forefront by the Apple Card.