Using a command, you can only put soul speed on boots, but in the anvil, you can put it on any armor piece.
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I don't know I made a video on it.
Thank you for your report!
We're actually already tracking this issue in MC-95555, so I resolved and linked this ticket as a duplicate.
However, that ticket has already been resolved. That means that this is not considered a bug and won't be fixed. Please do not leave a comment on the linked ticket.
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Cannot reproduce.