This key has been shared across forums like bbs.vefans.com and bbs.18wos.org for over a decade. It was often accompanied by instructions to "block updates to prevent the game from blocking the key if updated".
Here are some tips and tricks to help you get the most out of Euro Truck Simulator 2 version 1.1.1: euro truck simulator 2 version 1.1 1 product key
Some classic mods were built specifically for the early retail builds and were never updated for the Steam era. Can You Still Use Old Product Keys? This key has been shared across forums like bbs