This does seem like a weird bug -- but what's the use case? I don't think the url spec (http://www.faqs.org/rfcs/rfc3986.html) actually allows them there.
Is it used to figure out how much to auto link, for example, in a comment in HN or this very forum? That's certainly a valid usecase.
Similarly to this, when I'm writing Java, I use `final`^1 everywhere I can. It's nice to be able to know that anywhere later where the variable declared final is in scope, it will have the same value as at the point it's set. I don't need to look through any code to see if it's rebound; I know it hasn't been.
[1] "final" is kind of like "const", if I understand `const` right. `final int x = 3;` means that it is an error to later have the line of code `x = 4;`.
Yeah, if you have enough karma, you can see "vouch" links on dead submissions. The easiest way to see this is to go to https://news.ycombinator.com/newest; many of those links will be dead.