Ascot has long term disused platforms on the south / down side which I understand were mainly there for race day traffic. Ascot also has what used to be a double sided platform which is now fenced off on one platform edge - this may or may not count here.
Twickenham has disused bay platforms on the north / up side, which again may have been there mainly for additional trains for major rugby matches.
Clapham Junction has a disused platform (previously platform 1) at the northern edge. The platform edge is still there, the track hasn't been there as long as I can remember. The current platform 1 / 2 arrangement was made by altering the old platform 2.
Windsor and Eton Central has shrunk considerably.
Link
here to Windsor and Eton Central on the National Library of Scotland's digitised historic OS maps - this shows 1956 map - older maps are available but most areas have pre-1914 and post-1945 mapping available. Coverage is still being added to. (Thought I'd add a link for anyone who's not aware of it - you can move around and search by place name on that map.)