The journey planners are updated as far as I can tell. Normally there wouldn't be Southern branded trains between Victoria and Brighton, the standard timetable they run under the Gatwick Express banner. Thameslink also would typically be 4 direct trains to Brighton per hour from London Bridge which drops to 2 on Friday, journey planners might suggest a change to the Southern train making 4 trains which appears close to normal on first glance.
It's advisable to check on the day for any last minute cancellations on top of the reduced timetable.
