I understand that platform 12 hasn't been in regular use for some time, at least in part due to the platform not being on the paid side of the gateline. However, I am pretty sure I have seen the odd NT or XC train stabled there in recent times, or at least one of the times I passed through the station. I don't know if there are other operational reasons why the platform was taken OOU and it seems a shame to have lost a potentially useful stabling location.
I appreciate the need to accommodate longer trains in platform 11 so, as others have pointed out, T504 signal has been moved to avoid need to start past a signal. With the removal of the points this also prevents them being changed underneath a platformed train. However, would it not have been possible to retain the points and add some kind of interlocking, whereby a longer train occupying a given track circuit would prevent the points being thrown? Whilst the longer train would foul access to platform 12, could the latter not have been retained for stabling trains that have much longer times in the station than a train in the adjacent platform would? Just a thought.