My experience with plurals like they, them, etc. is that the people in question are utterly unknown, so these terms are used to apply to anyone without specificity. This occurs in user manuals and the like.
When you're speaking of a specific individual, either in print or verbally, there is no rational reason to use the plural. And doing so can be confusing, too. I'd be pretty angry as a firefighter if someone said "THEY are still in the burning building", or as law enforcement if someone said "THEY are still shooting", when what they really meant was a single person was involved.
People can live how they wish, but there's no justification for corrupting existing words in the language. Make up some new words if you must, but stop co-opting words and confusing normal conversation. I've gotten fed up enough with this nonsense that when someone aggressively insists I use "they/them" I tell them I'm very sorry that they ALSO suffer from multiple personality disorder. Sometimes I add "What is the name of the personality with whom I'm speaking at this moment?" "They" usually lose coherence at that point, to the entertainment of everyone in earshot.
"They" referring to a individual is quite appropriate in many circumstances, as in a comment like:- "Some random on Facebook is complaining that
they were banned from eevBlog!"
In that case, the person commenting has no idea whether the "random" is a man, woman, extraterrestrial, robot, or a dog.("On the Internet nobody knows you're a dog!")
In the above, I nearly used "you" in place of "the person commenting", but knew some nit picker would pick it up!
English varies quite a bit in what is a normal way of expressing things.
I read an American crime novel where someone says speaking of the victim,"I
knew her, so I dropped in to her house".
The detective then thinks "Egad!---This person spoke in the past tense, so must have known the victim was dead!
QED: They are the killer!"
To an Australian, the past tense would commonly be used in the case of "something done in the past", irrespective of the state of health of the person referred to, so the above deduction needs rereading to work out just what the hell is going on.
I might say, referring to a visit to another town. "I
knew the bloke at the Pizza shop, so dropped in to catch up"
The trip, obviously took place in the past, & the Pizza guy is still "hale & hearty".