Slicer Preview Releases, such as Slicer-4.11.x (rev 28560) are not archived indefinitely.
You can try using Slicer 4.11.20210226 (rev 29738) or Slicer 4.10.2 (rev 28257) instead. They can be downloaded from here.
Note that most (maybe all) features are available in the latest Slicer Preview Release, without installing any extensions, by using Data module to browse images and show/hide them and Segment Editor to segment them.
mpReview extension was developed to make the workflow more convenient for clinicians: a person responsible for data management set up the scenes that clinicians could process it more easily. There are discussions about reviving the extension in some form - you can contact @fedorov for more details.
@lassoan Deepa is working with me, and started looking into mpReview. The user instructions we had for the users relied on a specific version of Slicer (to provide consistency and stability, and not being able to keep updating the module to track Slicer updates). I was hoping Deepa can use that “known to work” release to get started, but looks like she will also need to explore more recent versions as well.
The goal is to preserve preview releases until a new stable comes out, but a cleanup may have to be started if the extensions server gets full. The extensions server got replaced around September, preview releases before that might have been discarded.
You can always check out a Slicer version and build it and the extensions if you want to check one specific version.