Note: This question is (in my opinion) not a duplicate of the question people have voted to close it for. I specifically cite that particular link, and am expressing confusion about how that answer in the link does not seem like it covers these close votes.
I am thoroughly confused as to what is allowed under the soft-question tag question. From the tag description, we have
"For questions whose answers can't be objectively evaluated as correct or incorrect, but which are still relevant to this site. Please be specific about what you are after."
but we also have in the Help Center "To prevent your question from being flagged and possibly removed, avoid asking subjective questions where … every answer is equally valid: 'What’s your favorite ___?'" (https://math.stackexchange.com/help/dont-ask)
So, naturally, it seems there is a fuzzy line in the middle, and this question seems to try to outline the difference between the two in which the punchline (from what I can tell) is:
"Between those two there are questions whose answers are not almost entirely based on opinions yet still cannot be objectively evaluated as correct or incorrect. These are soft questions that are not necessarily off-topic." (Are "soft questions" "opinion based"? How about vice versa?)
So, with that background, my question is the following:
What objective aspect determines whether a soft question fits math.SE? As working examples, why were the following three questions voted to be closed?
How to learn complex analysis? - The question specifically asks for recommendations based on experiences, and Help Center seems to OK that, and the question's answer is not just arbitrary. Also, the close note says: "Update the question so it can be answered with facts and citations. This will help others answer the question.", though people are answering it with names of textbooks, and is it not a fact that "Textbook X has helped me learn complex analysis"? Furthermore, the question: "How can one that has an basic knowledge of real analysis approach complex analysis? What do I start? Are there any books you would recommend?" seems very similar to the question "Which book provide a good and accessible introduction to number theory for a self-studying under-graduate student that already followed a course on abstract algebra but does not know any complex analysis?" @quid seems to recommend as an acceptable soft-question question. (given in Are "soft questions" "opinion based"? How about vice versa?)
Why do they write textbooks as lists of theorems and definitions, with unsolved exercises and proofs left to the reader? - A lot of the answers seems to explain the how giving the answers immediately could be discouraging, and they are long proses and again, I cannot find anything that these questions seem to violate.
The last one, the one that prompted me to write this question was the one that was this one:
and perhaps after a 'very minor' edit I made, I think the question fits within the boundaries outlined in the help center. Since this is the one that inspired me to write this post, I will check that this question fits the criteria in the help center:
To prevent your question from being flagged and possibly removed, avoid asking subjective questions where …
every answer is equally valid: “What’s your favorite ______?”: I would argue that you cannot answer this question with every answer: "I think the deepest, foundational, beautiful result in analysis is $1+2 = 3$"
your answer is provided along with the question, and you expect more answers: “I use ______ for ______, what do you use?” The OP does not know the answer to this question.
there is no actual problem to be solved: “I’m curious if other people feel like I do.” Perhaps this is the closest one, but the question asker is specifically asking for justifications, not a simple yes-no answer.
you are asking an open-ended, hypothetical question: “What if ______ happened?” No
your question is just a rant in disguise: “______ sucks, am I right?” No
It would be nice if you can suggest what edits could be done to any of these questions (if they are salvageable), so I can learn via examples.
Note that it seems soft-question tag has always been a bit controversial, since the only answer to Too many soft questions? has two comments that are discussing whether this website should be considered a "promo to mathematics" website, and both sides seem to have a noticeable (10+) amount of upvotes, so maybe a separate question on whether this website wants to be a "promo to mathematics" is also called for. (though I am not sure if that is too opinion based for Meta.SE)
(P.S. I have never asked a question in SE before (I just tried to emulate the style of questions I like answering in Math.SE by giving lots of examples and the relevant references), so feel free to modify the question in any way such as cutting paragraphs or bolding/highlighting the necessary, and feel free to answer with your own interpretation of the rules about any of the posts I linked, though my overall question is definitely a general question; these are ideally just some working examples.)