When trading with Piglins, they have a chance to drop an enchant book with Soul Speed on it. This is not a correct enchant book, and cannot be applied to boots because it is not a regular enchantment book.
To recreate this, just trade with Piglins until they drop the book.
Thank you for your report!
We're actually already tracking this issue in MC-174513, so I resolved and linked this ticket as a duplicate.
If you would like to add a vote and any extra information to the main ticket it would be appreciated.
If you haven't already, you might like to make use of the search feature in the future to see if the issue has already been reported.
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