[flagged]
Both, at the same time :)
ECS probably been around since at least early 2000s if not even in the 90s.
Edit: Wikipedia puts Thief as the "pioneer" of ECS:
> In 1998, Thief: The Dark Project pioneered an ECS. (https://dev.to/ovid/the-unknown-design-pattern-1l64) The engine was later used for its sequel, as well and System Shock 2.