Some list of serial killers or so (I cannot find it) also contained a plea not to expand it.

Yes, it was [0] the list of serial killers by number of victims, but since has changed [1] to say "You can help by expanding it with reliably sourced entries."

[0] https://i.imgur.com/hqiVtK6.png

[1] https://en.wikipedia.org/wiki/List_of_serial_killers_by_numb...