Speedrunning easy old maps will always be by far the most useful for 6s/general movement. Moving around cp_gullywash quickly has basically nothing in common with really any of the maps in this video, but maps like excavation, competitive (even though most of the maps aren't in rotation anymore), seven, ayers, and bob are great practice. Newer jumper mains tend to hate those maps, but that's really just because good movement is hard and they'd rather play something easy to go fast on.
Harder stuff can be good for getting consistent, but stickies do a lot of damage and there aren't many 2 sticky jumps in T4+ maps. Better to do harder routes in easy content.