Looks like my new "Children block" plugin, that uses Server Side Rendering to call core WordPress code, which was written in PHP ages ago, would not be listed because I also wrote another block called "parent" and included it in the same plugin. Had I merged the two blocks into one, and extended the "parent" part to do the full breadcrumb, then it would probably still be rejected because of the Server Side Rendering. Shame eh? https://github.com/bobbingwide/children-block