they/them has actually been in use in English when describing a single individual in a way that keeps their gender neutral or anonymous for a very long time.
this is not correct. "they" was not used to describe a single known individual to just keep their gender neutral or anonymous. it was used when talking about an unknown person or to keep the person unknown.
when talking about a person in a way to keep their gender anonymous, authors used phrasings that avoided pronouns or other things like the actually established gender neutral "he". there was no concept of simply using "they" to convey the meaning of an unknown gender. there was no need, as there was the gender neutral "he".
In English, when you deliberately use pronouns for someone that are not the pronouns they’ve requested, you are being intentionally harmful. You are pushing your view of the world onto them and likely implying that you either don’t care about or dislike their existence.
the person that requests the pronouns is also pushing their view onto others. if that is a bad thing, you should also condem the pronoun-requests.
pronouns do not exist to reaffirm one's gender identity. they are used to stand in for nouns. if you request special grammar rules for yourself, what does that make you? it is entitled thinking. i am special, treat me special.
it is just grammar. and here is the problem. pronoun requesters put value on grammar and try to get reaffirmation from that. grammar does not define your gender, it's the other way round. and i object the abuse of grammar for that. grammar need to be neural and not be seen as "disliking existence" when not using special requested grammar.
mind you, i am not talking about transsexuals using their new gender's pronouns. i am talking about made up pronouns and the usage of "they" - which is a made up pronoun in it's usage. i also object the new usage of "they" to convey unknown gender. you cannot even tell if the gender is unknown, the person is unknown, or if the person requested that pronoun or if the speaker is talking about a group.
a person with fluid or undecided gender could just as well request or rather default to the gender neutral usage of "he". having a gender is not special. so why should having a non binary gender be special.
To use a person’s chosen pronouns is to say “you are absolutely the man/woman/enby/etc. you say you are, and I care about you”.
this is literally not what is happening. you do not say pronouns to the person that requested them. you use them in conversation with other people. you do not get reaffirmation (i care about you) from the people you talk to. you just make them think about your gender, when they talk about you. which brings me to this again:
focussing on gender all the time is a step backward, not forward.