OxiPNG v9.1.3 introduced a new option for controlling the number of Zopfli iterations: oxipng --zopfli --zi=[1-255].
As you’d expect, using --zi=255 can save a couple bytes for some images. I haven’t noticed any size regressions from increasing the number of Zopfli iterations, so no new variants need to be added… thankfully.
In terms of the images found on OVERHEAD, the only improvement to note is the Deciphering Hugo series header image decreasing from 1849 bytes to 1824 bytes.