Call me paranoid, but I've always assumed urls are not permanent. Domains can change, usernames can change, and published things can be taken down.

For example, I may publish something as an online backup or a mirror of my real repositories, without any kind of guarantee whatsoever. ONLY sharing because I want people to be able to see how the things I do were made. I mean, even most FOSS licenses explicitly state that there are no guarantees of any kind.

IMHO I think people should stop with the "if it's on internet, it must retain compatibility" mindset, and instead switch to a "this will break or disappear at any time unless explicitly stated otherwise" mindset.