This references the character Ky Kiske from the "Guilty Gear" series.
The plural of Ky would be Kys which is an abbreviated form of kill yourself.
This is used mostly on Twitter/X by fans of the Guilty Gear series to avoid explicitly telling someone to kill themselves and may be accompanied by two photos of Ky Kiske.
Person 1: I think Pizza is pretty good with pineapple on it.
Person 2: Plural of Ky