I have to say this seems really weird since hir is like... his + her fused together, which just makes it seem neutral-sure-but-still-with-clearly-binary-influences? (and also can still sound very much like 'her', especially factoring in dialect and speech tendencies). And while it is by no means a comprehensive list, 'xie' is absent (albeit 'xe' and 'hir' are not) from Wiki's (presumably) common-use pronouns.I'm pretty sure xie/hir sprung up specifically out of non-binary circles, so it's always made sense to me that it's still kind of reigning.
Elverson first appeared in 1975 (which is the 'Spivak' set I still default to), but I can't seem to find a clear date on the others. It's amusing seeing how much 19th century (English) literature there is on the need for a neutral pronoun, though!