How do you modify the attack speed NBT in Minecraft 1.16?
Solution 1:
The following command was generated from MCStacker using the give command generator.
/give @p diamond_sword{AttributeModifiers:[{AttributeName:"generic.attack_speed",Name:"generic.attack_speed",Amount:1000,Operation:0,UUID:[I;-756158760,-1248440979,-1426007026,1776833309],Slot:"mainhand"}]} 1
This command worked for me. As for your command, it seems that Minecraft requires a UUID for attribute modifiers, instead of a UUIDLeast and UUIDMost.