I'm hung up on exactly this same thing right now, during our move to Aurora.
The core component that passes/exposes text key variables to the template doesn't have the value for message subject available as an option. The `IMMEDIATE_GENERAL` subject template, for example, only gets the Community Title passed in. As it is presently, you can't use anything else there 😒.
The `CONTENT_MENTIONS` and `MENTIONS` templates appear to be similar:
"templateSubject": "{{i18n \"ContentMentions.subject\" communityTitle=community.title}}\n",
"templateSubject": "{{i18n \"Mentions.subject\" author=notification.message.author.login communityTitle=community.title}}\n",
In both of these, you'd only be able to use the variables passed into the component.