KYS sounds harmless, but it is dangerous. Even if you meant to treat it as a joke, it can be a very sad read for someone. Someone with depression or mental illness may take it at face value and fall deeper into their condition. Encouraging self-harm, or even indirectly, is never okay because words, after all, have power. Harming language on the internet has implications on one's mental health. You can say every person should use this word carefully in chats.
Know more: https://radarro.com/kys-meaning/