When the movement efficiency attribute is set to anything above 0 (most easily visible at 1), not all blocks that impede movement are affected.
Soul sand and honey blocks work seemingly as intended; however, slime blocks, cobwebs, sweet berry bushes, powder snow, and any type of vines still impede movement when the movement efficiency attribute is set above 0.
Comments 5
Still in 1.21, and also affects the "movement_affected_by" predicate in data packs which doesn't consider these blocks as "affecting the movement".
In this version, the command becomes as following:
/give @p diamond[attribute_modifiers={modifiers:[{operation:add_value,amount:10d,type:"generic.movement_efficiency",id:"a"}]}]
Helper command: