It doesn't even have to be an "edge case", as stated in the article. My first name has a single accented vowel and breaks tons of online forms. What annoys me is not so much having to re-enter my name with the unaccented version of the vowel, it's when the form throws up an error message saying "Enter a Valid Name" [or suchlike]. I find that insulting. At least make your error text put the blame where it belongs: "So…
These kinds of limitations are fascinating because they reveal the cultural biases (and I don't mean bias in the negative sense) of their creators. A Western, American system might not be designed to accept names with accents and vowels because who has those in the States? Anecdotally, I also heard of a female games journalist who had trouble using a VR headset because she wore mascara. It threw off the eye tracking…
My guess would be on the order of tens or hundreds of thousands? It's 2016. There are very few good reasons left to assume text == ASCII anymore.