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.

Poisonous 1 causes the player to get just one poison counter when a Sliver deals combat damage to them, no matter how much damage that Sliver dealt.

A player with ten or more poison counters loses the game as a state-based action, even if no permanents with poisonous are on the battlefield.

If a creature has multiple instances of poisonous, each triggers separately.