What bit of “departure time” do people struggle with? It is when it departs!
Part of the problem is late advertising of platform. If an incoming train arrives less than 5 minutes before it is
due to leave, the platform will only be displayed for less than two minutes - regardless of how long it actually remains there. That is easily enough time for it to be missed altogether unless you give the screen your undivided attention.
The platform should in any case be displayed long enough before
actual departure for somone to get to it from any part of the station. The zoning at Waterloo - eg wait at platforms 1-6 - helps to reduce this time, but there are some destinations to which trains depart from both extremes, and due to unfortunate planning* are scheduled to do so at very similar times (Shepperton via Kingston at xx42 from platfroms 1-6, via Richmonvd at xx43 from platforms 20-24), similarly roundabouts at xx27 clockwise/xx30 anticlockwise. If you guess wrong which one is actually going to go first, by the time you get to the platform the train has gone, and by the time you get back the other may have gone (or at least dispppeared from the screen) too.
Kings Cross has a similar issue, since many destinations are also served from St Pancras Low Level.
Another issue with premature removal of platform number is people misremembering which platform was displayed by the time they reach the barrier - "was it 9 or 10". Done it myself at Waterloo - noted a train due to leave shortly as I come over the bridge from Waterloo East, make it to the barrier line only to then from force of habit join the train on platform 5 when today it's going from platform 6 (and that despite both trains still being displayed on the platform screens!)
*which incidentally means that although there are 4 trains an hour, at least in the peaks, to destinations on these routes there are 27-29 minute gaps.