Doomsday needs its graveyard and library manipulation intact to assemble its five-card pile and find Thassa's Oracle, so Leyline of the Void, especially free from the opening hand, exiles their cards before they ever hit the graveyard. That strips the graveyard of Brainstorm, Ponder, and other spells they cast, and anything they discard to Lion's Eye Diamond or Street Wraith cycling, leaving Doomsday fewer cards to build a pile from. It's a brutal mainphase hate piece against a deck with almost no proactive pressure. Barrowgoyf is a lifelink, deathtouch body that can block ground creatures like Orcish Bowmasters out of their sideboard, but it cannot block Dauthi Voidwalker because Voidwalker has shadow. It still has value with Leyline out, since your own graveyard keeps counting toward its stats. Hogaak and Grist come out since Hogaak relies on convoke/delve from your own graveyard, which Leyline doesn't restrict, but the slower grindy pieces matter less than pure disruption versus a glass-cannon combo deck racing to kill you out of nowhere. Trim Nihil Spellbomb as redundant graveyard hate once Leyline is in, and trim Pithing Needle because it has few meaningful targets: it can't stop mana abilities like Lion's Eye Diamond or Lotus Petal.