What happens if a player in my squad leaves the league?

Any non-playing player - be they injured, suspended, dropped, or transferred out of the league - does not accrue points. However, they do remain in your squad of 11 players until you replace them with someone else.
 
So, if, for example, a player in your squad leaves the league and you don't update your squad before the Saturday deadline, you won't gain any points from that player's performance in any other league.

However, because the player remains in your squad, you will still have a starting 11 (unless you removed someone else), so you will still gain points for the other, active players. In other words, you won't be penalised with no points for having an incomplete starting lineup.