TL;DR - We need a test run, a month, it is hard to say without actual experience.
Thinking about it, especially after Qami's post, it should be different for various sorting methods :
- Oldest first - it should be unpinned
- Highest votes - may be pinned
- Recent - probably unpinned
The reason is the desired outcome, which is important when we choose this or another sorting method.
When I sort in chronological order, oldest first, I intend to scroll and see if I have something to add, or if I am willing to post an answer to the question, while reading I vote answers. If I feel I have something to say then I do so, after scrolling through all answers, and sure I will notice the accepted answer as well if it is present.
Most voted first - the current behavior works in 90% cases, so a change here is not a huge deal, but maybe we can argue here that accepted on top may be important here.
Recent change sorting - one probably does expect to see that recently changed answer on top, maybe for the reason one prefers fresher ones
- other more dynamic reasons are not necessarily a factor because changes of sorting methods are not such a convenient procedure - so it from the start some long-lasting reason and sorting changes are rare for a person, I guess (at least for me).
And in this case, unpinning is the desired outcome, but there may be a value of comparison between fresh answers and the accepted one, having them next to each other, but it sure is for special people, but I can see how it can be useful for some adaptations and strategies for some people.
This way unpinning:
Fixes behavior for 100% questions with accepted answers for 2 out of 3 sorting methods.
Breaks (or maybe not - a matter of personal preferences) expected behavior for 11% of questions with accepted answers in 1 sorting method out of 3.
It looks like clear cut to me, in terms of consistency of results and meeting expectations. ("And" because we use and adapt our practices to known inconsistency, and if a change breaks something again, I guess we will do that again, adapt, and I do not think new adaptations in that case for those people are that much more annoying than existing ones)
Premise for 90 degree turn
In general, the reusability of old questions and answers is low on WB. I have an old answer 7yo on a different SE site, where I have one or few answers and it brings me some points once a half year or so(it is/was a recipe for a problem), on WB upvotes for my old answers as rare as that one q, I have 100+ answers, and even when I'm not most likable answer provider (far from it), it kinda leads me to certain conclusions, which I think I'm not wrong about. I think a similar situation is for other se sites which are not recipe providing ones.
Most of the activity happens in some period of TTL (time to live) for the question and then it is done, and that TTL correlates with the position on main. It also can be seen by the frequency of providing new answers to old questions.
90 degree turn
So until a question is on the main it has life, once gone it is toast and history. And for active users who are looking at active question's with an existing answer, they are probably curious about - what kind of answer op accepted. Or if they are looking to provide an answer then anyway the accepted one needs some reading (chronological sorting or not), and only with recent sorting unpinning may be an improvement. So for a fraction of questions, those which TTL isn't expired, and those which are important as they are currently in active use, it is less of clear cut than for older questions, mainly because of that curiosity aspect what kind of question op accepted, and after a few minutes of reading be a reminder that you reading a question with accepted answer, which could be forgotten already but here it is - a reminder.
So yeah, it needs a test run, as there are less obvious conveniences and inconveniences in the current situation. Be the change an individual preference, like button pin accepted (4th sorting button, which can be toggled on and off for each sorting method, creating 6 options in total), then that would be perfect as user experience, and we probably should insist on that option. Not caring about google traffic at all in this case and in this post.