If that's the case the planners probably won't sell a "route York" ticket on that train, though there's really no reason why anyone should buy one (you'd be paying more than double what you need to) so nobody has probably noticed.
Staff will probably accept them fine if you really wanted to, but again that's limited to people who can't refund and repurchase and in doing so save well over a hundred quid.
Just tried and Trainline will sell a route York ticket on the 0548, so presumably something has been put in to ensure they are issued. I don't know if there are any other examples we can try (with routed tickets) to see if this is still the general case, e.g. any tickets routed Didcot Parkway?
However I still suspect hardly anyone does because only people not in a position to refund their ticket would ever do that, because they'd be paying £100 over the odds.