(based on reading the license as a layperson and non-native English speaker)
No, you don’t have to fix a broken URI.
If the URI no longer points to the licensed material, you don’t have to supply it (nor any other URI that still/now does point to the material).
Section 3(a)(1)(A) uses this wording:
- ① retain
- ② if it is supplied by the Licensor
- ③ with the Licensed Material
① The word "retain" makes clear that you have to provide the URI as specified by the Licensors. So if they only specify
http://example.org/my-work, you have to use exactly this URI, not any other URI (strictly speaking, not even
② The word "supplied" again makes clear that it’s only about the URI which they specified themselves.
③ The words "with the" narrow it down to the URI that is specified in (or next to) the work. So you don’t have to look elsewhere to find a suitable URI.
Furthermore, nowhere does 3(a) require that you would have to keep your attribution updated. You have to make your attribution once, when you share the material. What the Licensors do after that (e.g., change their names, change their website’s domain, or even remove the license) is of no concern to you.
The only exception is 3(a)(3), but this requires you (if the Licensors request it) to remove parts of the attribution (e.g., to remove the broken URI), not to replace them (e.g., to add the new URI).