The Soul Speed enchantment operates solely when affixed to boots in the foot slot. This contrasts with the Depth Strider, Aqua Infinity, Feather Falling, and Respiration enchantments, which work on any item in any armor slot. This allows them to function on all 4 armor pieces elytras, pumpkins, and heads (all types of heads/skulls) as well. However, the Soul Speed enchantment doesn't share this flexibility. It appears that enchantments introduced in version 1.9 and later are tailored to specific items in their corresponding slots, unlike older enchantments. Through the use of an anvil, players can add Soul Speed to items other than boots as intended by MC-95555. This issue is also related to MC-268936.