Whats the intended use case for having this character in the first place? It seems like you could use it in a whole bunch of places to bypass regex/classifiers, or potentially a lot of things without proper encoding. I could potentially register at NelsonM<>inar somewhere and who would know the difference at a cursory glace?
Lots of services limit usernames to a certain range of characters, but some don't. However, there's tons of invisible unicode characters that you can use instead, so even without this one, the problem would still exist.
Thеrе arе many cyrillic lеttеrs looking idеntically to latin lеttеrs. In previous sentence all "e" letters were cyrillic, for example. So probably the only sane way is to limit username characters to latin letters.
Just like all the other spaces and zero width characters[1], it's originally for typography and typesetting. It is used to create word boundaries so that text would break at the right places in paragraphs, but without creating the visible, well, space between words.[2]
