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Plurality hasn’t been relevant since version 6G60 from 2010. Similar (but not identical) functionality was built into version 6L02 in 2014 and subsequent versions. But the I7 docs don’t provide the kind of detail on how it works and how one could customize it that Plurality’s docs did.
So Rewriting old code that uses Plurality by Emily Short is the only guide I know.
The Inform 7 Documentation and Resources post is a good first place to look for pointers, if overwhelmingly long. It includes a link to the above thread (tucked away in the Updating 6G60 section). You might want the other things in that section to follow the tutorial you’re using. But it also includes links to more current tutorials.