It was used colloquially, for sure... by rich corporate migrants that didn't want to self-ID as migrants. Or at least by the HR people and corpo consultants handling the international relocations and avoding the taboo word.
Which is what the previous post is saying and it certainly matches my experience as one of the "expats". I always self-identified as a migrant myself, though.
I've seen it used in media far more than in person. Mostly for soldiers that stuck around after wars wound down or wealthy people buying fancy homes in tropical locations.
The people I've worked with in tech from Scotland and England who have lived in the US long term but without becoming citizens don't even use the term. Honestly most people don't really use labels, just refer to someone in tech being 'from a country' when it comes up whether they became citizens or not.
We had enough of them at a time that "the expats" was a relevant group of people you needed to refer to for specific things. Language lessons, HR support, what have you. I definitely heard the anglo guys refer to themselves as that frequently, and that then became the word people used.
I had a chip on my shoulder about telling people I was a migrant, but I was pretty alone on that. The anglo guys mostly said they were "expats".
To be fair, they are pretty bad, unless you do like America and only allow people with certain skills and intelligence. Allowing refugees from war torn countries is very nice, but ultimately brings that war to the country itself and changes the culture into something very different.
No, it's because there's a lottery system that has been backlogged for decades. US immigration is such a bad process that skilled people go to Canada instead because the government there actually values those people.