Я всё понял, сам дурак. Правильная запись:
deb http://deb.debian.org/debian/ jessie main
deb http://security.debian.org/debian-security jessie/updates main____________________________________________
root@debian8:/etc/apt# cat sources.list
deb http://ftp.us.debian.org/debian/ jessie main
deb http://ftp.us.debian.org/debian/ jessie-updates main
deb http://ftp.us.debian.org/debian-security jessie/updates main
root@debian8:/etc/apt# LANG=C apt update
Ign http://ftp.us.debian.org jessie InRelease
Get:1 http://ftp.us.debian.org jessie-updates InRelease [7340 B]
Ign http://ftp.us.debian.org jessie/updates InRelease
Get:2 http://ftp.us.debian.org jessie Release.gpg [2420 B]
Ign http://ftp.us.debian.org jessie/updates Release.gpg
Get:3 http://ftp.us.debian.org jessie Release [148 kB]
Ign http://ftp.us.debian.org jessie/updates Release
Get:4 http://ftp.us.debian.org jessie/main amd64 Packages [6818 kB]
Get:5 http://ftp.us.debian.org jessie/main Translation-en [4581 kB]
Get:6 http://ftp.us.debian.org jessie/main Translation-ru [438 kB]
Err http://ftp.us.debian.org jessie/updates/main amd64 Packages
404 Not Found [IP: 208.80.154.15 80]
Ign http://ftp.us.debian.org jessie/updates/main Translation-en
Ign http://ftp.us.debian.org jessie/updates/main Translation-ru
Fetched 12.0 MB in 9s (1211 kB/s)
W: Failed to fetch http://ftp.us.debian.org/debian/dists/jessie-updates/InRelease Unable to find expected entry 'main/binary-amd64/Packages' in Release file (Wrong sources.list entry or malformed file)
W: Failed to fetch http://ftp.us.debian.org/debian-security/dists/jessie/updates/main/binary-amd64/Packages 404 Not Found [IP: 208.80.154.15 80]
E: Some index files failed to download. They have been ignored, or old ones used instead.
Я пробовал разные зеркала (fr, de, ru) — везде такая же ошибка.
