Can't find discobot welcome message text

The message you are looking for is made up of two parts. The first part is system_messages.welcome_user.text_body_template and the second part is system_messages.usage_tips.text_body_template.

For reference, this is the notification that is sent to new users when the discourse narrative bot welcome post type Site Setting is set to ‘Send all new users a welcome message with a quick start guide.’