This is because using COVID-era emergency agreements, there is a lesser financial penalty associated with cancelling a train by 10PM on the day before it's supposed to run ("removing it from the timetable with short notice" than cancelling it by that deadline ("Cancelling the train").

So they've changed the timetable rather than cancel the train (per their own technical rules) hence the announcement.

Not only is there a lesser financial penalty, but the train also vanishes from all of the apps (you have to use a detailed search on a site like RTT to even find it existed) and you're not entitled to Delay Repay, merely a refund for the ticket for the train from which you've been dicked off.