100% in favor of "they" as a singular. I have studied linguistics a fair amount.
I agree with
LizzyM that "they" as a singular pronoun is popular, but it actually always has been. Shakespeare used "they" as a singular - one of the greatest English language writers of all time. The use of "they" as a singular actually
fell out of fashion in the intervening time, and people decided it was a rule at some point that "they" is plural. Not starting and/or ending sentences with a preposition is also a "rule" that was just invented irrespective of actual common usage, even in very formal writing. Sometimes these "rules" get invented and then they later get viewed as more formal, when in fact the more common construction has always been common in all contexts.
It seems people feel a visceral
need to make "formal writing" different than what is comfortable in speech. They
need to have rules to obey that make it harder to write.
And then there are words that were never formal, like "dude", which is a cognate with "duds", which is an informal term for clothing. "Nice duds, dude. Sup with your job, bro?"