I know that Raspbian, being a Debian derived distribution, uses systemd as its init system, but the question is, why is there the binary /usr/sbin/service from the upstart init system? Is there to provide compatibility?
ps. Package upstart has never been uninstalled
It is there for compatibility. While the OS has switched to systemd, not all packages have been adapted for systemd, so some amount of backwards compatibility is available to run the older package scripts.
The service command is also helpful for backwards compatible with humans who expect the command to exist and work, even if it ends up restarting a systemd service instead of an Upstart service.
