Using "that" to refer to people

I saw a complaint about the supposedly increasing use of "that" to refer to people (instead of "who").

Since I have a digital copy of Jane Austen's "Emma," it took me only a few minutes to find an example of such a usage by Austen: Mr. Woodhouse "was a nervous man, easily depressed; fond of every body that he was used to, and hating to part with them; hating change of every kind."

But the Corpus of Historical American English also provides some evidence. I searched for the phrase "man who" and the phrase "man that". Amusingly, "the man that" seems to be decreasing in frequency. From the 1830s to the 1930s (by decades), it appears with at least 200 hits per decade, while since then it has been decreasing, with only 94 hits in the 1990s and 116 in the 2000s.

But the complaint was made by someone from Britain. The BYU corpora do not include a corpus of Historical British English like the COHA for American English, but you can use the BYU corpus site to search Google Books. And again, "man that" has decreased considerably since the 19th century: over 20,000 hits per decade from the 1830s to the 1900s (and up over 30,000 sometimes). Oddly enough, there is then a sudden drop in hits between the 1900s and the 1910s, from over 36k hits to about 12,500. And then it's been pretty steady since then.

So whatever is going on here, it is highly unlikely to be a matter of increasing frequency of use of "that" to refer to people (or at least to refer to "man"). This is what's known as "the frequency illusion."