When did the modern system of fixed key signatures become fully developed?

The modern system of fixed key signatures was not fully developed until the late 18th century.1

While the use of a single flat as a key signature dates back to the 11th or 12th century in early staff notation, the comprehensive system of keys and their associated fixed signatures evolved over several centuries. The concept of organizing musical keys and their corresponding signatures was further refined by tools such as the circle of fifths, which saw early diagrams in the late 17th century and a modernized version published in 1728. However, the standardized, universal application of the modern key signature system as it is recognized today reached its maturity in the late 18th century, coinciding with the era of composers such as Joseph Haydn.12