Shadow Sliver

• 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 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 Coast2021-03-19
  Official Ruling
  Multiple instances of shadow on the same creature are redundant.
• WWizards of the Coast2021-03-19
  Official Ruling
  Once a creature has been blocked, that creature remains blocked and will deal and be dealt combat damage even if it gains or loses shadow or if the blocking creature gains or loses shadow.
• WWizards of the Coast2021-03-19
  Official Ruling
  If an attacking creature has multiple evasion abilities, such as shadow and flying, a creature can block it only if that creature satisfies all of the appropriate evasion abilities.

Rulings via Scryfall