Since its launch back in 2013, Killer Instinct was known as one of the first few fighting games to launch with a Free-to-Play model.
Now, it's hard to call it much more than an extended demo due to the sheer amount of limitations that can only be lifted by actually buying the game, but still.
The free version will include a rotating free character, so it's a great way to try the game out and see if you like it. The character limit and their rotating nature would make it hard to actually play the game as if it was a F2P title though.
That said, Killer Instinct is one of the games where understanding fundamentals and general system mechanics is extremely rewarding, and learning character combos is a lesser issue thanks to the rather universal linker and finisher mechanic.
So, if you have any interest in KI, or have friends who do, you can now check the game out on Steam, free of charge. If you do end up enjoying it, the already low price and regular sales will also make it easy to jump in, just keep the crossplay limitations in mind.