What is bounty mode?

According to a post here, bounty mode gives awards for killing other players' commanders.

Is it a permanent boost to eco when you kill another players commander?

Yep! Also, if the commander dies via suicide or some other way that means no enemy unit killed it, the bonus afaik is awarded to the last player who damaged their army.

This is confirmed in this article:

With this mode enabled, when a commander dies the enemy that killed them gets a percentage bonus to their income, this bonus is set in the lobby. Someone self-destructs? Last person to attack their army gets the bonus. No more hiding, you fight or you die.