ECS was popularised with http://enwp.org/Dark_Engine in 1998. The architectural style probably existed already earlier, but I have no evidence I can point to.
Are you sure about the Dark Engine bit? I remember comments from people who've seen the code saying that it was an OOP mess with weird macros over C++ features that looked like COM on a bad day, which sounds completely antithetical to what people call ECS nowadays.
Perhaps it was confined to some part of the engine? ECS didn't come out of thin air after all, it is a generalization of practices people already used for things like particles, etc.
ECS was popularised with http://enwp.org/Dark_Engine in 1998. The architectural style probably existed already earlier, but I have no evidence I can point to.