I toured a few of the major distilleries in Kentucky last year, and most of them specifically mentioned that water is added after removal from the barrel, even for the less aged ones (of course this is skipped for some of the rarer products, e.g. E.H. Taylor barrel proof or Elijah Craig barrel proof). This is to meet certain product consistency requirements across their brands. I asked if it was the special “limestone water” originally used in distillation, and I believe it was Buffalo Trace that said no, it’s just regular water purified via reverse osmosis.