Just a little afterthought, following a conversation with a German speaker, and also chatting in Bulgarian.
Singular they are (my preference in this) could be looked upon as following a standard Indo-European language convention. In German you can be du (singular) or Sie (plural, but also formal or polite form); there are a couple of other possible words which could be translated as you in English, such as ihnen (Kann ich ihnen hilfen = may I help you), etc. In Bulgarian, there are also differences between singular you, and plural or polite yous, with -те referring to both they and you, depanding upon the situation.
In English, we don't have that. We only have a plural (or more correctly, I suspect, a polite) you: you are. It is possible to say you is, but only within certain dialects and patois. The use of they are for a single person could, I believe, be considered as following a similar rule. It is a demonstration, however unwitting, of good manners.
And, as said previously, it's nothing new, harking back to William Shakespeare (now there was one for writing gender-nonconforming characters) and beyond. Sorry to all for the musings on linguistics.