This was answered by a community member in a recent post.
Backer items will never be available to the public/new players in the future. They were items created solely for backers of our Indiegogo, which kickstarted and made Tower Unite a reality. These backers are part of the reason we have Tower Unite today, so they have all the right to brag. We spent a portion of the funds to develop the backer items for them exclusively.
We do have plans to have a speed shoe equivalent, but it will not look the same.