It is under Condition 4 of the T&Cs as shown in the leaflet,
also published online.
What about buying online? Who looks at the railcard when purchasing online?
As with lots of things regarding fares it is all inconsistent.
I tried to purchase 2 tickets for my partner and I for travel later in the week with senior railcards but I did not have hers with me only my own. The ticket clerk would not sell them to me.
I could however buy both tickets from the app on my phone and collect them from the TVM next to the clerk's window without either card present.
What is the point of checking the card when purchasing for the future? I can see that it is sensible to check for immediate travel, but you can show the ticket for the future then forget to take it or have lost it by the day of travel.
In 5 years of travel since having a senior railcard I have been asked to show it on very few occasions. An RPI at a block on a station asked for my ticket and when I said "why do you not want to see the railcard?" he said old people are not asked as they don't want to embarrass them!
I have also had the situation at Manchester Piccadilly where I was asked if I had a ticket. I said yes so you want to see it and the RPI said no that's OK.
It must be the older people are trusted more, I hope we all continue to maintain this trust.