The people who consider themselves the authorities on the subject are the least informed on the subject. They're basically militant agnostics: "I don't know, and neither do you."

Pronoun declarations are a declaration of membership of this cult of ignorance. Nobody in the past needed these declarations because sane people already had their own established method of conveying the same information - they'd just ask for your titles ("Mister") and not your pronouns ("he/him").

Specifically in science fiction, it sucks all the air out of the room that might otherwise allow more interesting ideas to be explored. You can't have an alien senator referred to using they/them pronouns to signify their position as representative of an entire population or because they're a collective consciousness like Mass Effect's Legion when you've already debased those labels to mean "I'm a left-wing narcissist who insists I'm a special snowflake."