Is "parent" supported in aliases? Is the "parent" key supported in Drush 9? It isn't in the example file here https://github.com/drush-ops/drush/blob/master/examples/example.site.yml or drush docs:aliases and I am trying to get a bastion working off of the Drush 8 docs (https://docs.drush.org/en/8.x/bastion/). 

Can anyone confirm it should pickup a "parent" key?

edit: I see here http://api.drush.org/api/drush/examples%21example.aliases.drushrc.php/master that "parent" was deprecated before the switch to YAML in Drush 9 so maybe it is called something else now. Not sure how to specify merging key values in YAML.
parent is not supported in Drush 9 aliases, but there are a couple of facilities that you can use.

Variables such as ${env.home} or ${foo.bar} can be used to inject environment and configuration values from Drush configuration into alias files.

An environment named common in any sitename.site.yml file will have its attributes copied into every environment defined in the same file.