Former stylistics and rhetoric teacher here ...
Using "they" as a third-person, indefinite singular should be avoided, as it can lead to pronoun-antecedent errors in your writing. This is especially true if we accept that "they" can be used as a singular for someone of unclear gender.
Example:
The knight and her squire approached the gates, where they saw some soldiers above on the ramparts. They threw open the gates, and rode into the city.
Who did? The knight? Her squire? The soldiers?
or
"I saw my friend sitting with a group of people at a table in the tavern. I approached, and they said 'hi John'"
who did? Your friend, the group? Someone in the group?
in the 14th century, or whenever, the use of "they", as an indeterminate pronoun, was almost always limited to a collective pronoun (plural) when denoting combined action: "The army approached the city, and before they reached the walls, they dismounted. When confronted with the insults from the men on the walls, they responded with one voice ..." (there are some exceptions)
Try to avoid pronoun use altogether, and use passive voice in some cases (the gates were thrown open). Never use a third person, indefinite pronoun, when the subject has clear gender. If you want to avoid "his or her" use "one" (one can choose any faction).
We can argue about the evolution and parameters of grammar all day, but this is about clarity and style. I am not going to read some semi-incomprehensible shit involving neologisms and pronoun-misuse