The Debian 9 (Stretch) repositories have been moved to archive.debian.org, after which the distribution will soon no longer be available through the main mirror network. The removal of Debian 9 from the main archive is scheduled for April 23rd.
The Debian 9 release was submitted on June 18, 2017 and was supported regularly until July 18, 2020, and in the LTS branch until June 30, 2022. As part of the Extended LTS program, updates will be published in a separate repository until June 30, 2027 to fix vulnerabilities in a limited set of Debian 9 packages for the amd64, armel and i386 architectures.
Source: opennet.ru