Currently it is not clear when it might be desirable to archive an extension or skin, outside of a few blindingly obvious situations such as its source code no longer being accessible. What other criteria apply, and when? Does it matter if an extension has been unmaintained for N years (and if so, what's a good cutoff for N)? If an extension/skin was never migrated to Git, or its maintainer(s) refuse to put it there, does that count in favor of archiving? And so on.
To start with, it would help to just document the criteria that were used to choose the extensions to archive recently as part of Projects-Cleanup . The information probably should be added on that project, since it seems to be the centralized location for extension/skin archival work.