There's no hard rules, though there is some good guidance in Why was my question closed as too broad, unclear, or opinion-based? (especially noting that unclear and too broad are often used interchangeably by people).
In this case, the cantrip question ("can cantrips affect the environment") is a mix of broad and unclear, in that obviously some do and some don't, but he really is asking about one cantrip, and his real issue is understanding that spells do what they say and side effects aren't RAW. Asking a more general question than you have throws up red flags to people and there's VtC, sometimes as broad and sometimes as unclear. You did a good job of answering it and covering all the bases, but when an answer requires multiple clauses about "what you think you're asking, what you are asking, and what you should be asking" it's a sign it's a poor question. Ask yourself, why does that question even say "cantrip?" Why isn't the question about any spell? Another muddy-thinking red flag.
The GM question, like many technique questions, was already closed once as unclear and then was reopened. It is just barely on the other side of that line now IMO - which is why many of the answers are a frame challenge.
Basically, both of these questions are on the line of "I'd vote to close in their current state if I were mildly irritable today." I haven't VtCed either (I did the GM one in its previous way more incoherent state).
You note the second has lots of answers like that's a good thing, but actually that many answers can be a sign that it is indeed too broad or unclear. In this case I think after one close and reopen people kinda just think "well, he's not going to get any more clear on this and that's actually his problem, because he thinks PCs pointing out logic holes in his plot is a bad players problem." So they reopened and are answering.
The cantrip one could have used some more clarification - if only because the clarification process often helps the OP actually get their thinking straight and gain clarity even before answers roll in. In the end, your answer is the "right" one most likely, but we like to teach a man to fish.
