When I use netcat to connect to an SSH port of a Debian system (
nc host 22), I got the response
SSH-2.0-OpenSSH_7.4p1 Debian-10+deb9u1. What does this
Debian-10 mean? The host is on Debian 9.
It simply reflects the Debian revision of the package currently in Debian 9; see the package page for details. The current version is 1:7.4p1-10+deb9u1; you can ignore the “1:”, then “7.4p1” is the version of OpenSSH, and “10+deb9u1” means this is the tenth release of the Debian package, updated for Debian 9 for the first time (more strictly, this is the first Debian 9 update following the tenth release of the Debian package of OpenSSH 7.4p1). The Debian changelog lists the changes made in the successive Debian packages of OpenSSH 7.4p1.
Put another way, “Debian-10+deb9u1” means it’s OpenSSH as packaged by Debian, and the Debian part of the version number (which is specific to the OpenSSH package) is “10+deb9u1”. The 10 there has nothing to do with the Debian release.