It's certainly stopped in the case of the Caledonian Sleeper; the new Mark 5 carriages have retention toilets like all other recent stock. I think the Mark 3A sleepers on the Night Riviera to Cornwall also came fitted with retention toilets from new in the 80s, but I wouldn't swear to it. Mark 3 day carriages definitely flushed onto the tracks, though those that are left are being retrofitted with tanks.

I always found the visible turds and toilet paper worse than the smell, but I haven't spent much time at Liverpool Street specifically.