Hello!
The Dying Light 10th Anniversary Bundle is free to own forever. It is a cosmetic DLC.
The expiry is unclear.
Released to celebrate the 10th Anniversary of Dying Light, this bundle includes the certified Hotshot Outfit, the invaluable Dawnbreaker shotgun, the radiant Sunburst revolver, and the glimmering Golden Idol machete. The gilded appearance of each item is a testament to everything you accomplished in Harran. This bundle also features the Golden Hour buggy paint job, symbolizing your role as a beacon of hope for survivors and a formidable force against enemies.
Get this DLC:
You will probably need to own the base game before you can claim. Dying Light was free at one point on Epic Games, so you may have it on that platform.