It is included as part of the Eggbot Inkscape extensions. If you have installed the Eggbot extensions for Inkscape, then you have it. If you have installed them but do not have it, then perhaps you have a much older version of those extensions? If so, then install the most recent set.
As to where to find it in Inkscape, it’s under the Extensions menu
Extensions > Eggbot Contributed > Spiral Wrapped Text….
