I think they're still there. Locations 32 and 33, by the toilets, on the "Terminal 5 Arrivals Ground Floor" page of the official T5 map: https://www.heathrow.com/file_source...row_T5_Map.pdf

They're the simpler water-fountain-tap sort, not the very expensive water treatment plant sort that is in the departures area by the top of the escalators.

The Costa staff on the landside just outside the customs exit will also fill a water bottle on request.