The Brief: "Is kil" is an internet slang term for "is dead" or "has been killed."
DEEP DIVE
Popular usages of this term include “Club Penguin is kil” or “Windows 7 is kil.” It can also be used to refer to “dead” memes and is often paired with a request to press F to pay respects.
According to KnowYourMeme, a correctly spelled version of the term originated from a 4chan copypasta about the death of John Lennon which included the phrase “John is kill.” Intentionally misspelled “is kil” memes are particularly popular on Reddit where a similar copypasta has been shared about Club Penguin instead of John Lennon.
Examples
Tsar is kil from HistoryMemes