Using the vanilla effect command, you can give a player a player a negative amplifier by intentionally causing it to overflow using:

/effect @p minecraft:jump_boost 30 253

This gives a nice negative jump boost effect making the player not jump as much (and 128 will lock the player to the ground)

However, doing this by potion effects/tipped arrows is not possible (and maybe potentially was in the past according to: https://www.planetminecraft.com/blog/how-to-make-players-unable-to-jump/ )

As the MobEffect class checks that the amplifier is > 0, else use 0.

The code should not be bounding this to 0 min, and should show jump boost in red if < 0, so that tipped arrows and potions can match behavior of whats possible with the effect command.