Climb like a spider in minecraft vanilla

/scoreboard objectives add shift stat.sneakTime

Adds a scoreboard called shift that increases when you shift

/execute @a[score_shift_min=1] ~ ~ ~ detect ~0.5 ~ ~ air 0 execute @a[score_shift_min=1,r=1] ~ ~ ~ detect ~-0.5 ~ ~ air 0 execute @a[score_shift_min=1,r=1] ~ ~ ~ detect ~ ~ ~0.5 air 0 execute @a[score_shift_min=1,r=1] ~ ~ ~ detect ~ ~ ~-0.5 air 0 execute @a[score_shift_min=1,r=1] ~ ~ ~ detect ~ ~1.9 ~ air 0 scoreboard players set @a[r=1] shift 0

Executes all the players with the score settled at 1 the action to detect around (all the blocks around at the same highness and one block up) and if successfully done, resets the score

/effect @a[score_shift_min=1] 25 1 1 true

Gives levitation effect to all the players with the score setted at one or more. If you want to climb faster, just change the last "1"

/scoreboard players set @a shift 0

resets the score, but only when the levitation effect has been successfully given