The American Heritage Dictionary has an updated its usage note for singular they .
My thinking about singular they changed in 2009 — a conversion experience — and has remained unchanged since. I think the pronoun is best used sparingly, and best avoided when one’s writing is subject to formal evaluation. Recasating a sentence can be a better choice than a singular they or a cumbersome he or she .
The most interesting part of the AHD note:
The recent use of singular they for a known person who identifies as neither male nor female remains controversial; as of 2015 only 27 percent of the Panelists accepted Scout was born male, but now they do not identify as either traditional gender. With regard to this last sentence, the Panel’s responses showed a clear generational shift: the approval rate was 4 percent among Panelists born before 1945 and 40 percent among Panelists born later.Singular they as a pronoun for a person who identifies as neither he nor she seems to me to inherently confusing. As I wrote in a comment on another they post, “If I were not a he and were making this kind of decision for myself, I’d choose singular pronouns.”