Thanks. Is that true only for American English or other areas too? I've only noticed this the last couple of years on HN. Before that "who" and "that" were used more carefully. Or at least I had the feeling it was. Sometimes I wondered if it's just whatever people's autocomplete happens to spit out first.

It's true for all of English, even historically. Ignore the grammar police. The differentiation between "who" and "that" in this particular context is extremely low on the list of things you'll ever need to worry about.