The mode knobs control the filter slope. "Legacy" in this case comes from one of the first versions of Misstortion 1, where the only available mode was a 12 dB/octave filter with a low Q value. I kept this specific mode because it was one that Headhunterz used a lot before I released Misstortion 2.
When you check the pre-dist clip filter option, the distortion is applied after the low pass filter (the clip filter) rather than before. This can be useful if you to put a focus on distorting certain lower frequencies without losing clarity in the high end.