On your turn, you use Sorin’s +1 ability, giving your creatures +1/+0 and lifelink until your next turn. You then attack with your Brimaz, King of Oreskos. Is the 1/1 Cat Soldier creature token affected by Sorin’s ability?

The way Sorins +1 ability works is that it it creates a continuous effect as the ability resolves. This effect modifies the characteristics of the creatures currently on the battlefield, so the creatures that it affects are locked in as the ability resolves.

This means that the creature token created by Brimaz is not affected by the +1 ability.

This also means that if you cast Act of Treason on your opponent’s Brimaz on your turn after the opponent activated Sorin’s ability, Brimaz will still have +1/+0 and lifelink while you control it!