I have been building design systems for years without knowing that was what they were called.
The name is only arriving now. Design system. Before that we said brand guidelines, or standards, or, at worst, a PDF nobody opened. But the thing itself is old. It is the answer to a question every organisation eventually asks: how do we stay ourselves across a thousand things made by people who have never met.
The work I am proudest of is not a poster. Right now it is a book of rules for the City of Edmonton's brand — pages that let other people make things that look like they belong. That is a strange thing to be proud of, and it took me a long time to understand why.
What I have learned is that a system is not a cage. The bad ones are. They tell you exactly what to do and nothing about why, so the moment somebody meets a situation the book did not predict, they either freeze or set the whole thing aside.
A good system explains itself. It gives you the reasoning, not just the ruling, so a person facing a new problem can extend the logic instead of breaking it. The test is simple. Hand it to somebody who was not in the room. If they can make something new that still feels like you, it is a system. If they can only trace what is already there, it is a colouring book.
Soon this will have a name, and talks, and a following. That seems right to me. It has always been the most useful thing a designer can leave behind, because it keeps working after you have gone.