Watcher Sliver

• WWizards of the Coast2013-07-01
  Official Ruling
  Abilities that Slivers grant, as well as power/toughness boosts, are cumulative. However, for some abilities, like flying, having more than one instance of the ability doesn't provide any additional benefit.
• WWizards of the Coast2013-07-01
  Official Ruling
  If the creature type of a Sliver changes so it's no longer a Sliver, it will no longer be affected by its own ability. Its ability will continue to affect other Sliver creatures.
• WWizards of the Coast2021-03-19
  Official Ruling
  Because damage remains marked on a creature until the damage is removed as the turn ends, nonlethal damage dealt to another Sliver may become lethal if Watcher Sliver leaves the battlefield during that turn.

Rulings via Scryfall