Updated:

1. Create the score

/scoreboard objectives add sneak minecraft.custom:minecraft.sneak_time

It goes up by one every tick while the player sneaks. Run it once.

2. Sneak to super jump

# Repeat, Always Active
execute as @a[scores={sneak=1..}] run effect give @s minecraft:jump_boost 1 4 true
# Chain, Always Active
scoreboard players reset @a[scores={sneak=1..}] sneak

Jump right after crouching to leap high. The reset makes the effect fade a second after you stop.

3. Item plus sneak

execute as @a[scores={sneak=1..}] if items entity @s weapon.mainhand minecraft:feather at @s run particle minecraft:cloud ~ ~ ~ 0.3 0.1 0.3 0 3

Only fires while holding a feather (1.20.5+). See held item effects.

4. Hold to teleport

execute as @a[scores={sneak=60..}] run tp @s 0 100 0
scoreboard players reset @a[scores={sneak=60..}] sneak

Teleports players once their total sneak time reaches 3 seconds. The score never goes down, so short crouches add up. Add a check for the block underfoot and you get an elevator that goes down when you sneak.

Troubleshooting

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