Is this an American thing? No one has ever in my life asked me "Who did you vote for?"
I have had plenty of people behave in a way that made it clear they assumed I agreed with them on political matters/issues that would have us voting the same way (sometimes correctly, sometimes incorrectly) but I have never been asked this question. Is it common or is it a contrivance in service of the article?
The only scenario where I believe people might directly ask "Who did you vote for?" is screening for dating. I don't know exactly how common it is, but I've heard multiple anecdotes about that being asked on dating apps or first dates, because they're not interested in dating someone who voted for Trump.
Prior to Trump it wasn't really a thing, because both parties were still following the law and maintaining a functioning democracy, so people could date across party lines and just agree-to-disagree about taxes or whatever.
My experience may not be representative, but I think it's very uncommon to outright ask "who did you vote for" in the US. It's more common (although many people still find it impolite or inappropriate in many situations) for someone to bring up an issue that is important to them and that strongly suggests a preference for one of our two viable political parties.