Six years ago, in 2012, I wrote a post where I made a continuum of examples of singular 'they' usage. Selections:

• unspecific ungendered: Someone has their head screwed on wrong.
• specific ungendered: I bet the cop who pulled you over was just trying to meet their quota.
• specific known-gender ungendered: The person who gave me a ride home from the dance last night doesn't take care of their car.
• unspecific gendered: Whenever I meet a cute boy contra dancing I friend them on Facebook.
• specific gendered: Look at that careless woman spilling beer all over their kid!

At the time I made the list I wrote:

I'm mostly fine up until the "unspecific gendered" referent, though the previous category can be a little iffy.

Since then I've looked back at the list every few years, and it's been a helpful way to track how my sense of 'they' has changed. In 2014 I wrote that some of the "unspecific ungendered" category sounded fine to me, and yesterday I noticed I was fine with the whole list.

Some things that I think have increased by comfort with singular 'they':

• When writing documentation, I've been using it consistently to refer to people, since they could turn out to have any gender. This is the top of the continuum.

• When writing interview feedback I use 'they' for the candidate, because I want to help hiring committee focus on the work the candidate showed me. This is the "specific known-gender ungendered" category, in the middle.

• I have an increasing number of friends who use 'they' pronouns.

Leah brought up that singular 'they' is generally more accepted for distal referents, something I wasn't thinking about when I wrote my 2012 post. That is, the farther away someone is, in time, space, relevance, etc, the more ok singular 'they' is. Here's another continuum of examples, all specific referents where gender is known to the speaker but not the listener:

• I got a call from my boss, and it sounded like they were calling through a tin-can telephone.
• When my boss gets in I can let them know.
• My boss is over there, with their shoes off.
• My boss is right here if you want to ask them yourself.
• This is my boss; they're also a contra dancer.

To me, the first two sound fine, the middle one I'm not sure about and might depend on distance, and the last two sound like I'm trying to indicate that my boss uses 'they' pronouns.

One example of this was a friend recently saying:

Emily [1] can bring the tablecloths when they come.

This is fine to me in a way that "This is Emily; they brought the tablecloths" would not be. [2] Emily isn't currently present, and they're peripheral to the current discussio.nno

Overall it seems to me like singular 'they' is continuing to expand in usage. I continue to think a good end state would be one where using it all the time for everyone is the normal thing, and you no longer need to guess people's genders from their appearance and presentation in order to refer to them.

(At some point I think I'll start pushing "they all" for plural 'they', but not yet. Ideally we can start adding 'all' as needed to disambiguate.)

[1] Not actually their name.

[2] Unless Emily used 'they' pronouns, of course.