kthresher binary package in Ubuntu Mantic amd64

 Tool to remove unused kernels that were installed automatically
 This tool removes those kernel packages marked as candidate for autoremoval.
 Those packages are generally installed via Unattended upgrade or
 meta-packages. By default the latest kernel and manual installations are
 marked to Never Auto Remove.
 .
 thresher - A device that first separates the head of a stalk of grain from
 the straw, and then further separates the kernel from the rest of the head.

Source package