OK, I looked it up on Wikipedia. The bishop and queen were the last to have their moves set changed to the modern form in the 15th or 16th century. But even since then there have been some tweaks, such as the 3 move and 50 move rules for draws, and the orientation of the board. So you could maybe argue no balancing since the 16th century, and only a few bug fixes after that.
Has also maintained an active playerbase for 1500+ years (startrek.website) angielski