SirPrimrose

@Jiingy yes please fix this. I have just discovered this issue on my PvP server and it is entirely game balance breaking. Client side mods can easily abuse this to turn slow swinging weapons/tools into near-instant cooldown weapons. Feel free to contact me for bug testing/fixing. 

I'm less speaking to the fact the amplifier in unsupported and more saying that people would not be trying to misuse it if a better method existed, especially for something so conceptually simple as stopping a jump. I do not run command blocks on my server so I instead have implemented a mechanic to cancel jump movement when it is detected, however even though you claim "the client doesn't even move up/down", this is in fact untrue in my case when you can only control the behavior from the server side. Cancelling jumps results in jerky movement that increases depending on the ping of the player.

Side note: The original topic of this ticket in particular has to do with the excessive amounts of knockback applied to a player that also has jump boost, which I think is a bug and unintended side effect of jump boost, regardless of whether the specific amplifier is supported. 

I would like to voice my concern over the dismissal/supposed duplication of this issue, specifically with jump boost. If you are unaware, many plugin developers and map builders lack an official way to disable the player from jumping. This is a seemingly simple problem that has no real resolution, other than setting a very high Jump Boost effect. However, as seen by this ticket and others like it, this causes undue side effects of rather large knockback when being hit and still being able to gain small amounts of speed by jumping while sprinting.

The Minecraft community at large has not discovered a way to disable jumping on the client's side, only how to mitigate it's effects through server code or command blocks. High jump boost gets the job done in some situations but has the aforementioned side effects and is not "clean". It would be great to see the ability to control the player's movements in the future, such as enabling/disabling sprinting, jumping, swimming, etc. without needing to resort to hacky solutions like potion effects.