Hi Justice,
When you say that the burndown chart "is not recognized" (as an artifact), then you are specifically referring to one particular agile methodology: Scrum. Here it is true that according to the current Scrum Guide only three items are recognized as "official" artifacts: Product Backlog, Sprint backlog, and Increment.
However... Scrum is only one of many methods. To mention two others: The PMBOK Guide 7th edition mentions the burndown chart as an artifact, and agile (in general) uses the term artifact for it as well.
Therefore: Widen your view, and it becomes an artifact.
