Last modified: 2014-09-04 02:58:34 UTC
When someone writes text:sub(4, 8) it uses string.sub(text, 4, 8). Wikis widely use Unicode, so it's not a good idea. It should really use mw.ustring.sub(text, 4, 8) instead. While this will add a bit of overhead, it is a much safer default and less likely to break characters in half.
I think there were portability concerns with doing something like this.
Considering Gerrit change #48781 undid something like this, I think it's fairly safe to close this as WONTFIX.