Following the install instructions at https://roots.io/sage/docs/theme-installation/ still results in composer downloading Sage 8.5.3. I have run composer clear-cache
to make sure I have cleared the cache for previous versions.
Reading through https://getcomposer.org/doc/03-cli.md#create-project, it looks like I can specify a minimum version of the project using some flags, but this may be something the team wants to update so that it installs Sage 9 by default instead of the older Sage 8.5.3.