The Jargon File (version 4.4.7, 29 Dec 2003):

pr0n
 //

    [Usenet, IRC] Pornography. Originally this referred only to Internet porn
    but since then it has expanded to refer to just about any kind. The term
    comes from the warez kiddies tendency to replace letters with numbers. At
    some point on IRC someone mistyped, swapping the middle two characters, and
    the name stuck. It then propagated over into mainstream hacker usage. New
    versions of the Mozilla web browser internally refer to the image library
    as ?libpr0n?. Compare filk, grilf, hing and newsfroup.


The Free On-line Dictionary of Computing (30 December 2018):

pron
pr0n

    (Or "pr0n") B1FF-speak for pornography.  Often seen
   on IRC in such desperate cries for help as "I WNAT
   PRON!!!!!"

   (1997-09-14)