For most of human history, majority of population couldn't read.

Common misconception. Most people couldn't read and write in Latin, which was the only literacy statistic anyone cared about in the middle ages. People could usually write out something in their mother tongue in order to write a letter or leave a note. Keep in mind that spelling was not standardized in English until basic education (as you alluded to). That's when rules about grammar and spelling started to be standardized. Prior to that, people just wrote phonetically and there was no societal norms for that being wrong. As long as you could be understood, you were writing.