Comment by int_19h

If you can deal with Russian, this is listed as a "full database" on ru-wiki:

http://gramoty.ru/birchbark/

I don't know if something similar exists in English.

There are plenty of notes written by peasants and servants, so it clearly wasn't just an elite thing. But IIRC the vast majority of those letters are from the city proper, and Novgorod Republic was a city-state governing a large territory outside of its walls, so we're talking mostly about the urban population.

Keep in mind also that this is an actual republic with all male citizens voting in a public assembly on important matters, so it would make sense for the culture to appreciate the trivium more universally.