A tradition started by Joseph Strong (according to Ari Rahikkala's memory of history anyway). Shirithians generally use the zie form (see Wikipedia for specifics). The use of gender-neutral pronouns in common posts and official documents is one of the more regrettable idiosyncrasies of the Shirerithian language.
An example, from the Imperial Decree founding the position of the Ombudsman:
"b. No candidate may nominate zirself, although any candidate may declare zir unwillingness to participate."