It’s not in the core. So you’ll need to explicitly add it as a dependency in your web-resources declaration, something like:
<web-resource key="Sample-resources-url" name="Sample Web Resources">
<resource type="download" name="my-js.js" location="/js/my-js.js"/>
<dependency>com.atlassian.auiplugin:aui-table-sortable</dependency> is the important snippet here).
The link the @nmansilla gave has the availability status in the top of the document (some are in ‘core’, some like this one - are not).
Sortable-table has been out of experimental since 5.8+ (which is what all “modern” Atlassian products are using - see https://bitbucket.org/atlassian/aui-adg/wiki/versions/product-version-matrix ).