I'm not in favour of pop-up messages to annoy all those who don't need reminding. Just get rid of the autofill until a technical solution is found. The issue almost entirely hurts young people who can be under immense financial and other pressures.
The sites I buy from - the Forum's site, and SWR. Neither have autofill and it doesn't bother me although I use a Railcard on almost every journey.
The mild inconvenience is well outweighed by the serious impact it has on a lot of people (we will only be seeing a tiny minority). And the impact is a lottery too, depending on when that fatal request to see the Railcard occurs, whether it's after a few days or a few months.
The industry could solve this swiftly, if enough pressure was applied.
The existing question on the ticket sites that's implied (not explicitly asked) is "do you have a Railcard"?
These people answer that by saying, "yes I do".
What's wrong is that the Railcard has expired.
Some sellers know when the card has expired, because they sold it! But they still autofill, brainlessly.