Attrition hasn't been implemented because it is impossible to do. The closest thing we have is how you lose units during a Crusade or Jihad, however this only works if the army is a Crusade or Jihad army, and its rather unsubtle. There are mechanics that we can use to represent this attrition. In EBI your army loses morale whilst campaigning in foreign lands to represent the loss of men, etc. This is controlled by certain factors, such as the generals own traits (how good he is at logistics for ex.).

Foot