It was asked in 2014 and remained on-topic for 4 years before being migrated. How did it suddenly become off-topic in May, 2018? I don't remember seeing any discussion on meta regarding this. Did one of the mods of this site unilaterally decide to migrate it with no input from the community?
No, it didn't happen on its own. Here is how it happened:
21 Jun 2014: Former moderator senshin suggested that migration would be better:
@Null It is possible that you would get better answers to this question at History. If you would like your question to be migrated there, please flag your post, select "other", and in the text box, state that you would like your post migrated to History.SE. If not, feel free to leave it here and see what answers you get. – senshin Jun 21 '14 at 9:35
14 Feb. 2018: You edited the title to clarify that question is looking answer from historical point of view.
14 Feb. 2018: You flagged NAA to this & this answers. - Flags were disputed.
7 May 2018: You again flagged both answers - Flag was marked helpful.
8 May to 12 May we, moderators discussed on this issue, we also consulted History Stack Exchange moderator to make sure whether the question fits on History Stack Exchange or not.
Meanwhile you also discussed it on chat-room:
sv. on 11 May 2018: @TheDestroyer What if the majority opinion is wrong? If those mythology-based answers are allowed we will now have two questions which are not dup. and yet the answers are same. This doesn't make sense. Not good for the site. If you decide to keep those answers, I'd recommend closing the question itself or migrate it to History.SE. Accepting the question and then allowing mythology-based answers is unfair.
On 12th May 2018, we asked CM to migrate and the post was migrated to History Stack Exchange.
Now, let's discuss why we migrated the question:
Reading the body of question, it is clear that OP wanted to know the historical/archeological point of view as he is talking about Indus valley and Aryan theory. This type of questions can not be answered by scriptures and from Hinduism point of view. Since this is dealing more with history (not Puranas or Itihasas scriptures of Hiduism) than Hinduism, it can be answered well on History Stack Exchange or Mythology Stack Exchange. That's why probably couple of answers failed to answer the question properly on this site.
Though we have "Historical questions about the evolution of Hinduism" as one of the on-topic subject at help center, we should make sure it is properly answerable on Hinduism Stack Exchange. If we allow questions to be answered from historical/archeological point of view then it may contradict with answers posted from Hinduism/religious point of view.
Among the other posts you have mentioned, two questions are asking for a scriptural support, this sounds a cultural question, Were Aryans the forefathers of Hindus? would be more suitable on History Stack Exchange. These type of questions needs to be handled considering the scope and answer-ability.