True.
Not modern trains, but there are still plenty of 1980s trains with openable flap windows around. Not big enough to stick your head out, but enough to throw rubbish out if you're determined. In that neck of the woods, they have just retired Southern's Class 455s which had opening windows and ran in south London (SWR still operate them into Waterloo). The Class 313s on the Coastway (Portsmouth to Brighton, Seaford, etc) are the only others operated by Southern, but don't get anywhere near London. Plenty of others, diesel and electric, at other operators.
Theo