Will Ubuntu 18.04 LTS still support Upstart or do we have to change to systemd?

Title pretty much says it all, I'm aware that since Ubuntu 15.04 systemd is the default service management tool, but can we still use Upstart with 18.04 if we'd like?

If we can still use Upstart with Ubuntu 18.04, has Canonical announced a specific release where Upstart won't work any longer, or are we on our own to try and use Upstart with current/future Ubuntu versions until it doesn't work in a particular case?

Will Ubuntu 18.04 ship with Upstart in addition to systemd, or do we have to install Upstart separately?

Before somebody marks this as a duplicate, I realize this post:

Upstart or Systemd

states that Upstart was used through 14.10 and that now systemd is the default, but my question is can we still use Upstart with 18.04 even if it's not the default.

Since Ubuntu 17.10 Ubuntu has dropped support for the Upstart init daemon in the default Ubuntu repositories. The latest version at http://upstart.ubuntu.com/, Upstart 1.13.2, was released on September 4, 2014.