How did it happen that Artscroll et al refers to Nusach Polin as Nusach Ashkenaz, and Nusach Ashkenaz is called Minhag Ashkenaz?
If you look at a siddur from 100 years ago it will say Nusach Polin or Lita on it and if it says Ashkenaz that means it is German. Nowadays if a siddur says Ashkenaz it is actually Nusach Polin (maybe with a little Lita thrown in.)
When I tell people that I daven Ashkenaz they think I mean what Artscroll calls Ashkenaz.
How/when did this happen?

David

In regard to Minhag Polin, I believe that is the nusach of Great Britain (Minhag Anglia). The nusach Artscroll publishes is Nusach Lita as far as I can tell.

When did Minhag Lita drop the recital of piyutim? Did they ever recite them?

Rallis

The selichos for ב‏ה''ב and תענית צבור found in the Artscroll siddur are most definitely מנהג פולין and not ‏ .מנהג ליטאAs for piyuttim in ליטא, the מנהג in all Ashkenazic regions was to recite פיוטים. For examples in ליטא specifically, we know from מעשה רב that the גר''א recited many פיוטים, and it is recorded that the Yeshiva of Volozhin recited מערבית on the nights of Yom Tov.

Melech

At the beginning, in the Ashkenazi custom, there were slight variations between Minhag Ostreich and Minhag Reinus. Minhag Ostreich spread out to Poland, Hungary, Russia and Lithuania, and every place east to the Elbe river.
Minhag Ostreich got the name "Minhag Polin" since the printers wanted to sell their books of Piyyutim etc., so they wrote in the first page the places these Piyyutim were said, and Poland was mentioned first because it had the biggest population, and this way in peoples memory the name became "Minhag Polin".
Since there were two types of Machzorim, when the printers wanted to sell Minhag Reinus Machzorim they called them "Ashkenaz" since this was the Minhag used in Ashkenaz (Germany) Switzerland and France.
When the Chassidim started to use the Chassidic Nusach (which they first called Nusach Ha'ari, then Nusach Sepharad). At that time they started calling Nusach Polin etc. also Nusach Ashkenaz, since it wasn't Nusach Sepharad but was nusach Ostreich which was one of the Ashkenazi customs, therefore at the 19th century you can find in Poland that the non Chasidic Nusach is called "Nusach Ashkenaz".

Michael FRBSH