So I must have customized it at some point, but I’d like to change it again. After reading Changing Welcome Message I have searched for this welcome message but cannot find the exact one:
Pick a phrase from the message you would like to edit and type it in the search box. You can also tick the Only show overridden checkbox to help you narrow it down.
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.’