I'm surprised you haven't seen one of the several threads where this was explained (example).
At the time of the migration to the Discourse forum framework in 2021, something had to be done with all the posts in the database that had been created by accounts that had since been deleted (either at the request of the user or by a moderator). The people handling the migration decided to give ownership of all those posts to the @system user.
Established accounts that are deleted post migration according to Arduino's account deletion policies are "anonymized", which does not transfer their posts to the @system user, but instead changes their username to a randomized string with the prefix anon.