Compare https://matplotlib.org/stable/tutorials/colors/colormapnorms.html

vs https://matplotlib.org/devdocs/tutorials/colors/colormapnorms.html

The same thing happens with the custom norm at the end of the tutorial. I'm not sure about TwoSlopeNorm, but at least for the custom one, the 0 is definitely not supposed to be in the middle. This half broke when merging #20054, with the 0 tick at the old place but the 0 colour in the middle. Then with d1c5a6a, the 0 tick also moved to the middle.