I have attached an excel file of historical classifications by income from World Bank. But it contains only for fiscal year from 1989 to 2018.

According to WB, the low-, middle-, and high-income group thresholds were established in 1989 based largely on operational thresholds that had previously been established.  "... the staff report 'Per Capita Income: Estimating Internationally Comparable Numbers,' presented to the Executive Directors in 1989, established an explicit benchmark of $6,000 per capita in 1987 prices."
I guess that's why they don't have data before 1989.

Hope that still helps!

(sorry the pc does not allow me to type in Chinese :(((

Relevant link:
https://datahelpdesk.worldbank.org/knowledgebase/articles/378833-how-are-the-income-group-thresholds-determined