Currently we have no index support.  Our index support for Markua is
specified but not implemented, and we have nothing in Leanpub
Flavoured Markdown.

The short answer as to why we haven't done this yet in the past 7+
years is that indexes are for 2 main things:

1. Helping readers find things.
2. Selling paper books (people look at the index).

#1 is solved mostly by being able to search the ebook (especially as a
PDF), and #2 is not very relevant for Leanpub.  Our book landing pages
do the job of trying to sell the ebook.

Now, we do recognize that an index does add some value, especially
when a print-ready PDF is being exported.  So, that's why I specified
it in Markua, and why we will end up building it.  But it will be one
of the last things we build in our Markua support, so chances are this
is between 9 and 18 months out.

Finally, details of what is currently not yet implemented in Leanpub's
Markua support is here:

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