It will have an impact on Forest Hill; I use this line a lot and rely on it to change at Clapham Junction for journeys to Surrey and Hampshire.
I've always found it saves time avoiding central London and Waterloo.

Very sorry to hear this - I'd heard rumours, but I thought the idea was to replace it with the ELL to Clapham, although I'm not sure this will be a comparable service for those who want to travel direct to Victoria.