Something interesting I learned in my speech class textbook, Reflect and Relate (Second Edition) about they/them/their in the English language, page 187:
A language’s regulative rules also change. When you learned to speak
and write English, for example, you probably were taught that they is inappropriate as a singular pronoun. But before the 1850s, people commonly used they as the singular pronoun for individuals whose gender was unknown—for example, “the owner went out to the stables, where they fed the horses” (Spender, 1990).
In 1850, male grammarians petitioned the British Parliament to pass a law declaring that all gender-indeterminate references be labeled he instead of they (Spender, 1990). Since that time, teachers of English worldwide have taught their students that they used as a singular pronoun is “not proper.”
I always thought that “they” was considered grammatically improper as a singular pronoun because of the possibility of confusion with the plural “they” (an additional factor also explored in this paper), although it is also worth noting that you/your/yours is used in the singular and plural, generally without confusion, frequently. I wanted to see if their were other sources on the above and there are! I was interested to learn about this in particular because, time and time again, I’ve heard people rudely remark on they/them/their being “improper” after someone would mention this as the pronoun set they’d like people to use in reference to them – both online and off. They/them/their is also the pronoun set I have gelled with best in reference to myself. If anyone has more info on this, I’d love to know about it!