209
xkcd #3184: Funny Numbers
(imgs.xkcd.com)
Title text:
In 1899, people were walking around shouting '23' at each other and laughing, and confused reporters were writing articles trying to figure out what it meant.
Transcript:
Transcript will show once it’s been added to explainxkcd.com
Source: https://xkcd.com/3184/
I was going to say, I think the perpetuation of leetspeak and most of its use falls squarely into the millennial generation's early 90s into the early 2000s.