Regardless of your movement speed attribute, your horizontal speed while falling and the speed at which you can move underwater will not change. As a result of this, when speed is slower than normal, jumping while moving makes you go relatively faster, but if it is faster than normal, jumping will make you go much slower.
Note: This is very noticeable when used together with scale attribute, i.e. if you make yourself a 16th of your size and speed and jump height and then move forward while jumping.
To reproduce
Run the following command to decrease your movement speed:
/attribute @s minecraft:generic.movement_speed base set 0.0001
Jump while walking
Swim in the water
To reset to regular movement speed, use the command
/attribute @s minecraft:generic.movement_speed base set 0.10000000149011612
Expected behaviour
The horizontal movement speed in the air and in water would also be affected by the movement speed attribute
Observed behaviour
The horizontal movement speed while in air or in water is the same as normal
Related issues
Comments

Please update the description to include steps to reproduce, as well as expected VS observed results.
Please also attach a video if possible showcasing the issue

Steps to reproduce:
/attribute @s minecraft:generic.movement_speed base set 0.0001
Notice how when jumping you are able to move way further than you should be able to.
This is basically just MC-2112, since Horizontal fall speed is falling.

Thank you for your report!
After consideration, the issue is being closed as Working as Intended.
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relates to MC-2112