I don't think they're referring to expired cards. They no longer ask for expiration date (like they did years ago), so I don't think they can even know the cards are expired. I think it's something else (but I'm not sure what).

Is there anything "non-standard" about the specific cards they emailed you about? Like, for example, did they used to allow something other than MC/Visa/Amex/Discover (since that's the only 4 they now accept for "add card")?

Even if it's not simple, there's a lot of information that can be gleaned from the first digits of a card number. They can figure out, for example, which bank issues it, and if they hate Bank Zebra for some reason, they can disallow all cards issued by Bank Zebra. They can even get more specific, I think, if they wish.

What are these emails titled? I don't think I've gotten any such emails (and, btw, I have tons of expired cards in my various RN accounts).

Oh, I just noticed you seem to be based in Australia (you show you Location as MEL). Maybe they no longer accept cards issued in certain countries (which they can figure out from the number itself), and that's what they disabled?