Runaway

It's rather impressive that Seth McFarland had a joke that would have been offensive at ANY time in the last 200 years. Or certainly at any reasonable setting for his Old West tale.

Because certainly Mr. McFarland is concerned his jokes aren't historically correct.

(Sure, the campfire scene in Blazing Saddles was impolite and not acceptable for children, but not offensive in the same way. Incidentally, Googling those two words ("Campfire Scene") yields mostly Blazing Saddles results.)

More of a setup than a joke (and evidently used more than it could really support), there is a town fair shooting gallery that shows runaway slaves as targets. Even without checking on when the film is actually set, it has to be in the 19th century. While there may have been pioneers in the area before 1800, this is when the West really came into being. In any case,  the "joke" would have been problematic in many areas.

Considering the outcome of a war that killed more Americans than any other - and by far the highest as a percentage of population - it would have been in bad taste certainly from Good Friday 1865, and probably as early as April 12, 1861. Even when the Awakening began around the 1880s and the Lost Cause narrative began taking hold, to ascribe such barbaric motives to that Holy Cause might have been unpalatable even in the South. (Plus it would have tainted that narrative's concept of slavery as a benign institution.)

Even before the start of the American Civil War, slavery was a subject that most of the country didn't have a sense of humor about. There were Southerners who considered it part of the heritage, abolitionists who thought it ungodly, and everyone else who could see where the debate was going to end up and not necessarily in a hurry to push it there. One can easily imagine John Brown showing up to do what Django does here. After the passage of the Fugitive Slave Act of 1850,  helping runaways became a badge of honor in much of the North.  And before that, while constitutionally required, there was no particular advantage to interfering with fugitive slaves. Which means the reference really only makes sense in slave states before 1850.

Actually, it's set in Arizona in 1882, so it really makes no sense at all. Slavery hadn't existed for 17 years. Even Doc Brown is three years too early.