This is by no means a new question — the first question you linked was asked by me, over two years ago, about the exact same topic. My personal stance remains that the decision to include spoilers should generally be left to the answerer themself, and edits that are only meant to insert spoiler markup without improving any other aspect of the question should be rejected.
I generally have personal internal guidelines for when to put a part of my answer in spoilers. They might have evolved over time, but in general I follow two rules:
1. I only put something in spoilers if it's not the entire answer.
In essence, I follow badp's policy on spoilers.
This means that if the entire answer or effectively the entire answer excluding some auxiliary framing text would be placed into spoilers so that the answer wouldn't make any sense without them even as a hint, I will leave the whole thing as is. This precludes answers such as "The answer is [rest of answer under a spoiler]" or even "We have [spoiler] and so we can deduce that [another spoiler] to arrive at the conclusion that [final answer spoiler]".
You might mention this answer or this answer as a counterexample, but in both of those cases, I put the final answer on top, and the explanation under spoilers, because it's really the rule that the sequence follows or the reasoning behind each word that's a challenge for people to find out — the answer itself is just to check if your rule is correct.
So far people have been good about not making edits to put an entire answer behind spoilers just because they think it's a good idea to spoiler everything that explains an answer. And I don't edit people's answers who do do this to use smarter spoiler formatting, because I understand people have different norms about this stuff.
2. I only put an answer in spoilers if the formatting of the answer affords it.
This is one place where I might insert spoilers contrary to badp's policy.
One categorical example of questions that do afford a spoiler-by-default format are questions containing a series of mini-questions in a list (a few of which I've answered here). Those ones, I don't mind putting spoilers on because it's natural that you might only want to see the answer for a few of them at once.
For other examples of things that might be spoilered that aren't list questions, take a look at this edit that I just made to an answer of my own to insert spoilers. That part of the answer was already isolated in a quote, to draw attention to it, so that can be put into a spoiler.
However, some of the edits I've seen have literally just taken the single answer for a question and put it on its own line to spoiler it out, like this:
We have some interesting explanation here, and the answer is blah, as we expected before.
We have some interesting explanation here, and the answer is:
as we expected before.
This ruins the formatting I intended for that answer, all for the sake of hiding the answer from somebody who may or may not want to see it.