The End-of-life (EOL) for a major release of GitHub Enterprise server is generally one year following its release date.
For example, GitHub Enterprise Server 2.16.0 was released on January 22, 2019 and had an EOL date of January 22, 2020. Similarly, 2.17.0 was released on May 23, 2019 and had an EOL date of May 23, 2020.
GitHub’s Releases page can be used to get these dates, as well as release notes for any given version. EOL notices are also published at the top of documentation pages for their respective version, as well as in the Release notes for patches leading up to EOL.