|Being an engineer, I’m familiar with problems and hate to tempt fate by saying something is going well. “So far, so good” is therefore one of my favorite clichés.On Stackexchange I learned there is an archaic meaning of “so” that means “in this manner/condition.”
FreeDictionary says the idiom was first recorded in James Kelly’s Scottish Proverbs (1721), where it is defined: “So far, so good. So much is done to good purpose.” The title of the book implies the phrase was already well known.
Etymonline offers a detailed etymology of “so” and agrees with the 1721 reference. So it seems Kelly was the first to write the phrase down and publish it.