Till what period was poultry considered pareve like fish, and not meat?
It is hard to pinpoint a historical date when the custom of considering poultry as meat began.
The Talmud records a dispute concerning whether it is permitted to eat poultry with milk, and in the place of Rabbi Yosi Ha-Glili it was custom to do so (Chullin 116).
Note that according to Tosafos (Chullin 113) this actually involves a Torah prohibition; the Rambam, however, writes that somebody who claims that it is forbidden to eat poultry with milk on a Torah level transgresses the prohibition of adding to the Torah.
Ethiopian Jews know of the prohibition of mixing meat and milk, but not of the extension to poultry. This is understandable, because their community is generally dated back to the First Temple era. As noted, it is hard to know when the custom of prohibiting poultry and milk became universal.