For future reference, you can use
http://packages.slackware.com/ to search for individual files within packages. Just change the dropdown to "content", pick your version, and put in the search string.
In this example, since lsmod is a symlink and not an included file in the package (it is accomplished by the doinst.sh script within the package), you'd have to search a little harder since you won't find a listing for it in /bin or /usr/bin, but the kmod package does list a manpage for lsmod, so reason would lead you to use that package to get lsmod back.