Using a repeating minecraft command block and executing the command:

/item replace entity @a armor.feet with leather_boots

Leather boots will appear on the player. If taken off in creative mode by shift clicking a few times, the boots are no longer visible. Upon death, the visual bug is fixed.

With a command like: /item replace entity @a armor.feet with leather_boots{AttributeModifiers:[

{AttributeName:"generic.movement_speed",Amount:1,UUID:[I;-1,-1,-1,-1],Slot:"feet"}

]}

The attribute is still applied even though the boots are not visible. This only occurs in creative mode.