Immerse Gamepack for Dying Light has a 14-day free trial, then a one-time payment of $19.99 USD for a lifetime license. A lifetime license is good for as long as you play the game. There is no free version after the trial ends.
You don't need to cancel your free trial. Because no payment details are collected, the trial expires on its own at no cost to you.
When your trial ends, Enhanced Headphone Mode turns off completely. To turn it back on, buy a license.