The fact that the $6,000 cut-off in 1989 ($12,616 today when updated for international inflation) assured that all the countries previously classified as “industrial” were re-labeled “high income” does not get us very far in understanding the meaning and relevance today of the $12,616 threshold, or even the relevance of $6,000 in 1989.
So I am none the wiser on the answer to my original question: Why $12,616? Your website claims that there is a “stable relationship with poverty and infant mortality.” That would be interesting to see, but users of your website can find no documentation. Every year (for many years) these income classifications are revised with essentially the same minimal and puzzling documentation. And believe me the attention these classifications get is not just from “analytic users.” They have huge influence.
If there is really nothing to these classifications that the Bank has produced for decades then it would be good to finally tell users. And, as I say in my reply to Shaida’s post, while the review you refer to is welcome, it is surely unwise to keep updating and publishing these classifications when you have not yet sorted out how they should be done.