@frankiesaxx That'd be interesting to get a real look at. I suspect we're exposed to far more causual writing now than in quite some time.
As of 1800, only about 25% of Europe's population was literate, and earlier than that rates may have been as low as 5% in rural areas. The culture of letter-writing itself had to be developed, and use of postal mail was a fairly newish tradition in the 19th century, for the general population. Books gave specific guidance on style.