I understand and support Colin's stance that the default configuration
shipped with Debian should follow upstream.
The nasty thing about subsystem directives is that they cannot be
overridden by a .conf file placed inside the /etc/ssh/sshd_config.d/
folder, due to this bug:
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=998834
So if one wants to use e.g. the internal sftp server, one MUST modify
the /etc/ssh/sshd_config file. This in turn interferes with automatic
upgrade scripts like cron-apt, unattended-upgrades etc.
I thought about moving the subsystem directive into a new file inside
the /etc/ssh/sshd_config.d/ folder. However, this would result in broken
configurations on machines where the administrator keeps a modified
/etc/ssh/sshd_config file during an upgrade.