Phyrexian Swarmlord's last ability is that you create infect tokens based on the number of poison counters on your opponents. Let's say I'm playing a 1v1v1 and I have him on the battlefield with opponent A having 2 poison counters and opponent B having 9, and I proliferate. If I choose player B's poison counters as part of my proliferate that player loses from having 10 poison counters, but as the game continues, would I still get 10 tokens from them where the player lost with that many counters on them? Or should I exclude them from proliferate as long as possible to continue getting 9 infect tokens because they need to be alive in the game to get the tokens?
You will not get to count the counters from the player who has lost.
When a player loses the game in multiplayer, that player leaves the game, which means they are not in any way a part of the game. They are no longer an opponent, they no longer have poison counters, they do not exist at all as far as the game is concerned.
800.4. Unlike two-player games, multiplayer games can continue after one or more players have left the game.
102.1. A player is one of the people in the game.
102.3. In a multiplayer game between teams, a player's teammates are the other players on their team, and the player's opponents are all players not on their team.
So a person who has left the game is not "in the game", which means they are not "a player", and "an opponent" must be "a player".
If your opponent loses the game because of having 10 or more poison counters, they leave the game and Phyrexian Swarmlord doesn't 'detect' them anymore; you won't get Insect tokens from them. So if your goals is to create many tokens, it's best to keep them alive and not target them while proliferating.