Official answer: I have no idea why “foraging” wouldn’t be a legitimate topic. I don’t see it on any list, including disabled topics, so this isn’t making any sense to me. Can you link me to the question you used it as a topic on? That way I can view any topic editing that was done to the question. Now I’m really interested to know where that one went.
As for why topics get removed/disabled in the first place, that varies. Sometimes, as in the case of “mods”, they basically get merged into the usual word for the topic (in this case, “moderator”.) It can also happen due to a misspelling, because it’s a joke topic, it’s too long (like a whole sentence), or is so esoteric that it isn’t common enough.