Right now, it is a cultural marker. If you run a linguistic use analysis, you see that it is used on the coasts and a few key urban areas in the US and it hasn't made progress toward universality.
Do you have any citations? If you mean the deliberate usage of singular they as a neuter pronoun for subjects who identify as non-binary, then sure. But otherwise, singular they is used by almost everybody some of the time. always4getpass' comment, at least on its face, refers to the latter usage.
Frankly, I think even people offended at being forced to use the former usage will relent. Refusing to accommodate (rightly or wrongly) is confrontational, and being confrontational is exhausting. It's already quite natural to use singular they when using a passive, impersonal voice, and using a passive voice is what people will invariably resort to when they're exhausted.
Except you know Shakespeare using singular they[0]. Then the fact that I’m willing to bet decent money that random individuals in rural areas also use singular they all the time without realising. It is just when a trans person wants singular they exclusively that it becomes an issue.
I'm surprised that this misses the distinction made by signalsmith above: the way Shakespeare uses "they" to refer to someone is unremarkable, but what strikes many present-day speakers as an error (even if they fail to explain why) is the use of it to apply to a particular person, especially a named person.
Frankly, I think even people offended at being forced to use the former usage will relent. Refusing to accommodate (rightly or wrongly) is confrontational, and being confrontational is exhausting. It's already quite natural to use singular they when using a passive, impersonal voice, and using a passive voice is what people will invariably resort to when they're exhausted.