This question refers to the rule "Don't Feed the Trolls". I want to know where this phrase originated (I know where I saw it first but I'm curious if that came from)
Would a question like this be on topic here? Is there a better suited SE site?
I think questions about the history of moderation, including how some terms arose and spread, are on-topic. They should be scoped to communities. For example, "how did 'trolls' arise as a term for malicious users?" would be on-topic here and would not be answered by merely pointing to the first folkloric/mythic use of "troll" in that sense. On the other hand, a question about the origin of "troll" as a word for certain people would be on-topic on ELU and might be satisfied by that folkoric use.
The key difference is that our questions should be scoped to communities, while word-origin questions on ELU are scoped more broadly. If you want the broad answer go there; if you're interested in the community-specific phenomenon, ask here.
Is the History of Moderation on topic?
Yes, I believe, moderation history is on topic here.
I want to know where ["Don't Feed the Trolls"] phrase originated.
This type of broad question question is more suited for English.SE. However, when posting there you need be explicit with your question: "Where was X first mentioned". You need to show evidence of research, even if it didn't turn up anything useful. In this case, quoting where the you first saw it would be useful.
As Monica mentions in her answer, if you can narrow the scope of your question to online communities, this would be a good question for here.