The subTheme drush command is currently not picked up by Drush 9. The blog post by Moshe Weitzman describes basic steps for porting existing command.
https://weitzman.github.io/blog/port-to-drush9
I have tried to follow the blog post but so far i have been unable to get this working. If i succeed i will provide a patch.
Adding Drush 9 support should not break backward compatibility as the original drush.inc file can co-exist.
Update
The latest patch should make the commands available in Drush 9. The current beta8 version of Drush 9 does not handle drush commands for themes, only for modules. There is a patch available for drush here: https://patch-diff.githubusercontent.com/raw/drush-ops/drush/pull/3089.p...
Needs review
9.0
Code
Not all content is available!
It's likely this issue predates Contrib.social: some issue and comment data are missing.
Here is a patch for the composer script approach.
But realizing i think this whole approach should be obsolete now? 📌 Making a theme compatible with core's theme generator is too difficult Needs work