At CommonLit, we intentionally use the singular "they" when discussing the speakers of poems. This is because readers should not assume the speaker of the poem is the author. If the speaker's identity is not explicit, it can be guessed at but not assumed. By using singular "they" (or "the speaker") when discussing poetry, we encourage readers to look beyond author biography to how a speaker is characterized in the text.
Exceptions to this rule are made when the speaker's identity is clear (such as in "On the Death of Anne Bronte," when it is clear Charlotte Bronte is writing from her own point of view about her sister Anne's death).
We adopted the singular "they" after it was named The American Dialect Society's 2015 Word of the Year, and we've found teachers and students have quickly adapted to it once the reasoning behind it use was explained.
You will also see singular "they" and "their" used throughout the teacher website when referring to one student instead of "him or her" or "his or hers."
Different sources of the history and usefulness of singular they can be found online in the Oxford English Dictionary, Merriam Webster's words to watch, Time Magazine, and The Washington Post.