Historically, Project Gutenberg organized its content by the year
Project Gutenberg published each work. 

For example, "etext98" contained all the books released in 1998.

By the early 2000s, that approach was problematic because so many
files, with somewhat difficult to interpret filenames, went in each
annual folder. 

The new system is to use numbered folders. You can find a description
of this approach in the GUTINDEX.ALL link on the "offline directories"
page of www.gutenberg.org

Over time, all the etextXX content was migrated to the numbered 
locations. Years later, only a few items remained that do not seem
to be represented elsewhere. They might not be part of any Project 
Gutenberg eBook.

They have been relocated to this "retired" location.