A reference to circus performers “walking the slack line” is made in the novel “Hard Times” by Charles Dickens published in 1854. There is no mention of how this line is rigged or what performers did on the line but possibly more info could be obtained from periodicals from this era.
I have had a small amount of experiensce with slack lines, and just for the record I think I will stick to unicyling. (way safer)
Slacklining probably outdates tightropes. All you really need is a rope attached between two trees and you’ve got a primitive slackline. You’ll probably want to find information on modern slacklining, the kind invented by rock climbers.