A link for the lazy

https://store.steampowered.com/app/577940/Killer_Instinct/

~ Some info ~

Upon the release of our 10th Anniversary Update, there will be two versions of KI:

The Killer Instinct Base Game will be FREE TO PLAY on all platforms: Xbox consoles (One, Series X|S), Windows PC, and Steam.
    The free KI base game will feature one free weekly rotating Fighter and Single-Player, Local, and Ranked modes.
The Killer Instinct Anniversary Edition ($29.99 USD) will include all 29 Fighters, and all Premium Content ever released: the VIP Double XP Booster, holiday accessories, and more.

Also -

On Steam:
    The current purchase-only Steam version of Killer Instinct will be replaced by Killer Instinct Anniversary Edition. All purchased Steam versions of Killer Instinct will be upgraded to the Anniversary Edition for free.
    Additionally, the Permanent VIP Double XP Booster and Gargos’ Golden God and Argent Shroud colors will be available on Steam for the first time as part of the Anniversary Edition.

Fight for Free! Play various game modes with a free weekly rotating fighter, including online multiplayer. Get the complete collection by Upgrading to the Anniversary Edition to unlock all 29 fighters, retro costumes, premium accessories, and more from Seasons 1-3!

So this is more of a “base game is f2p with full game presumably the same price as it was before” thing?

I’m pretty sure that it used to cost money.