we should have all the answers for all the versions in one question,
rather than a separate question for each version of macOS
In short, yes.
Apple changes things with each revision of their hardware and software; some of it unintentional (aka ahem..."features") and lots intentional (aka "stuff that annoys users to the nth degree").
Simply stating that a question is for a different version and/or product doesn't negate it from still being current or relevant.
In the question you originally asked, you state (via an edit):
The duplicate question how to quickly reboot from OSX to Windows and
back does have one answer that may work for macOS Sierra,
The key element that would disqualify it from being a duplicate is if the solution presented didn't work; which would be more than significant enough a factor to not be flagged.
Looking at this from a purely logistical point of view, having the same question for each version of macOS would be rather large. From Mavericks (10.9) to High Sierra shipping today (10.14), that's 5 different versions (not including the minor upgrades).
Why would we need 5 different questions/answers for the same thing if the question/answer are still relevant?
Going further, if the question is "different" because of the hardware being is is older further adds to number of permutations. Just doing a cursory check the MBP goes back to 2006 and has about 20 major versions (i.e. Mid-2011) excluding the sizes (13", 15", 17") and options (i.e. Retina, TouchBar, etc.)
Would we need to have 100 different permutations of the same question/answer because the hardware/software combo is older/different? I think it would be illogical and inefficient to have a separate question for each possibility even if limited to only the major releases of hardware/software.
Reopening Closed Questions
As this is community driven, we do make mistakes and close things for many reasons including marking questions as dupes when they are in fact not. For instance, in this question asked within the last 30 days, it was marked as a dupe but reopened because the OP added the following clarification:
This isn't the same question as the proposed duplicated, at all. I am
not suggesting that anything is eating my disk space. I'm also not
saying that restarting it changes anything. I'm stating that the disk
util says I have 300 GB free, which is likely the case given what I
deleted, and the OS says ~90 GB free, an amount that did not change as
I was deleting things and did not change when restarting the
system...so basically the freed space was not given back.
As is the case with your question. It's entirely possible that your question isn't a duplicate, but until you rule out the solution(s) already provided for the previously asked question, it's a duplicate.
Updating the previously asked question
Being a community site, it's encouraged to edit questions/answers to make them more accurate (without trying to change the author's intent). For example, if the previous question/answer was (not) a solution to your issue, you could edit the answer with a single line saying:
Update as of some date: This is confirmed to (not) work in mac OS Sierra (10.13.x).
Asking a new question wouldn't be considered a dupe at that point and you would be contributing back to the community by keeping the original question relevant.
To address your comment:
So your suggestion is essentially to try every one of the many broken
answers in the dupe and then ask a new question if none of them work?
The Help Center addresses that concern:
Have you thoroughly searched for an answer before asking your
question? Sharing your research helps everyone. Tell us what you found
and why it didn’t meet your needs. This demonstrates that you’ve taken
the time to try to help yourself, it saves us from reiterating obvious
answers, and above all, it helps you get a more specific and relevant
If the answer is works or is broken, you can up/down vote and even add comments (i.e. "This answer doesn't work for whatever reason")
The key is to reasonably attempt to research before generically stating a question is may not be relevant because it's perceived to be obsolete.