This is deb.debian.org. This service provides mirrors for the following Debian archive repositories:
/debian//debian-debug//debian-ports//debian-security//debian-security-debug/The server deb.debian.org does not have packages itself, but the name has SRV records in DNS that lets apt in stretch and later find places.
To use it with a sufficiently recent apt, you for instance can put
deb http://deb.debian.org/debian unstable main deb http://deb.debian.org/debian-debug unstable-debug main deb http://deb.debian.org/debian-ports unstable mainin your
sources.list.
As of July 2022 the SRV record is
_http._tcp.deb.debian.org. IN SRV 10 1 80 prod.debian.map.fastly.net.If you hit the server behind
deb.debian.org directly, either because you use an older apt or because you use a HTTP proxy that does not support SRV records, your requests will get HTTP redirected to one of the CDN instances.
This service is sponsored by Fastly.