Leetspeak
Swapping letters for numbers and symbols — 3 for e, 4 for a, 0 for o — born on 1980s BBSes as both a filter dodge and a badge of membership.
Leetspeak (from elite) replaces characters with graphically similar digits and symbols: elite becomes 31337, hacker becomes h4x0r. The style grew up on warez- and phreak-adjacent bulletin boards, partly to slip past crude word filters and searches, partly as a signal that you were one of the initiated.
"Elite" originally meant the protected areas of a BBS that only trusted members could reach. The writing survived the dial-up era and lives on in gaming and IRC culture, these days mostly as a joke.