> These URLs always refer to the latest definition and won’t break if the file is edited, unlike links to a specific line number.

You can always get the URL on GitHub for a particular line number at a particular revision which won't break when the file changes. A persistent link to a function can break in a different way in that maybe the function, while named the same, changes behaviour in a way that it no longer does exactly what it originally did. It's not obvious to me which of these breakages is more relevant.

When you talk about "hackable" URLs it would be great to be able to get a URL for a named function at a particular revision. This solves both problems. I have an immutable reference to a particular piece of code, but then by hacking the URL I should be able to still see the most recent version.