From the webpage

  Note added January, 2022: This document was written for Go's release in 2009, and has not been updated significantly since. Although it is a good guide to understand how to use the language itself, thanks to the stability of the language, it says little about the libraries and nothing about significant changes to the Go ecosystem since it was written, such as the build system, testing, modules, and polymorphism. There are no plans to update it, as so much has happened and a large and growing set of documents, blogs, and books do a fine job of describing modern Go usage. Effective Go continues to be useful, but the reader should understand it is far from a complete guide.