“You’re on the list.” It was first used in a Dora the Explorer and BTS horror story AU by @filmfics on Twitter. The “list” was a harvest list of people she would kill for blocking/interacting with her. You can use this against people you hate for fun, not in some murder type way.
YOTL is from a DoraxBTS crack au created by filmfics on Twitter. It is an abbreviation meaning "You're on the list". You would typically use this when someone says something you disagree with.
Person 1: Have you seen that au yet?
Person 2: Yeah, it was kind of boring. I didn't like it very much.
Person 1: YOTL
Wow, I was at the accordion music festival and when I went to the restroom this Street Yodeler approached me wanting to suck me off for my loose change!