It may safely be assumed that today's literacy rates are unprecedented: Unesco reports that "Since 1950, the adult literacy rate at the world level has increased by 5 percentage points every decade on average, from 55.7% in 1950 to 86.2% in 2015" (Reading the past, writing the future, Paris 2016) - and most of the 1950 total represents European and north American countries with rates in excess of 90-95% (Unesco, World literacy at mid-century, Paris 1957), a condition that certainly doesn't apply in earlier centuries.

There's really no reliable way to measure global rates before the 20th century, except to say they were lower than today's or even the 1950 level - inevitably given the spread of education in the 19th and 20th centuries. Even in Europe, barely half of the population seems to have enjoyed functional literacy as late as 1850, and fewer still could read earlier in the century. The US seems to have been a 19th-century leader with extensive schoolong, but even there a fifth were illiterate in 1870, more than a tenth in 1900.

Rates doubtless had their downs as well as their ups, as conditions for written communication became less or more favourable in particular periods, as reflected in the varying availability of written sources over time. But the long-run trend has certainly been upward with the spread of writing itself and later of general basic schooling and printing.