Muscle 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
  Muscle Sliver's ability does give itself +1/+1.
• 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 a Sliver may become lethal if Muscle Sliver leaves the battlefield during that turn.

Rulings via Scryfall