It means to not do anything stupid and stay safe. It is definitely not an acronym for kys.

When the “crush” has no intentions of taking things further, but they like the attention. So they flirt here or there, send dm/texts just to keep the person interested, knowing damn well they’re staying single.

people say this in order to inform someone to "kill them selves" without receiving any consequences from using the harsh term since both phrases are long for "KYS"