The singular 'they' in English first appears in a.. (checks notes) a 1375 sexy romance about furries and a werewolf

everything here appears to be in order, carry on

https://public.oed.com/blog/a-brief-history-of-singular-they/

http://www.eleusinianm.co.uk/middle-english-literature-retold-in-modern-english/romances/william-and-the-werewolf

via https://twitter.com/Jessifer/status/1092147228003782656

You think I am making this up but

<< And the strongest animal, the most formidable, is the bear. If we were able to steal two bearskins and put you both inside them, there is no one alive who would challenge you on the highway! I can think of no better plan than that you disguise yourselves as bears; for they, of all animals, are most like people.' >>

<<'See how I have fared!' she exclaimed. And Alexandra began to dress Emelior first, fastened her in the skin with strong cords, all about her proper clothes, so that, in all honesty, no man would think anything other than that she was a bear, so exactly did each piece join with another. Emelior was delighted and said: 'Dearest Alexandra, how do you like me now? Am I not a handsome bear?'>>

<< Alexandra then wrapped William in the other bearskin. And when he was properly sewn in, William said happily to Emelior: 'Tell me, my sweetheart, how do you like me now?'

'By Mary the mild Queen of Heaven, so strong and fierce a bear you seem to be that I am terrified to look you in the face!'>>

<<William and Emelior made their way quickly through the garden, fiercely on their four feet, as bears do, and happened to encounter one of the new arrivals from Greece, looking at the plants. Terrified out of his wits, he raced out of the garden as fast as feet can run, for he firmly believed that they were pursuing him and would eat him. >>

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<< So ends the first part of this tale. All who would like to hear more should offer an 'Our Father' to the High King of Heaven for the noble Earl of Hereford, Sir Humphrey de Bohun, nephew of old King Edward who lies at Gloucester. For he is the first to have this tale translated from French into English for the benefit of all Englishmen; whoever prays thus, may God grant him bliss! >>