Sims 4 will be going free on all platforms, including Steam, from October 18th and onward.
How to claim: more information on that date.

Source: https://www.ea.com/en-gb/games/the-sims/the-sims-4/news/play-the-sims-4-for-free-beginning-october-18
https://www.indiegamebundles.com/the-sims-4-for-free-from-october-18th/