For "decommissioned pages" as in deleted pages that no longer exist, the correct response is 404, page not found, and this is exactly what will happen in mojoPortal.
For cases where the page still exists but has a new url, we do not want a 404 for the old url and use of 301 is meant to solve this problem.
In mojoPortal we use short urls based on the title of the page and since we don't reflect the site hierarchy in the url, you can freely move pages around without changing the url and this gives us a freedom to move pages around without affecting established page ranking or breaking bookmarks that visitors may have made.
In the case where a page is renamed and the url does change, for example if I have a page named Product List with an url of /productlist.aspx and I later decide for some reason that I want to change the name to Product Catalog and the url will change to productcatalog.aspx. The way mojoPortal handles this right now is that the menu and the site map (both the user site map and the google site map) and all links for the page generated by mojoPortal will now reflect the new url /productcatalog.aspx, but the old url /productlist.aspx still works and does not break, so we have no 404 and no broken bookmark in this case. We merely de-emphasize the old url and promote the use of the new url.
Now I suppose it would be possible if we decide that we would rather have a 301 for renamed pages with changed urls if the url that brought us to the page is not the current url we could implement logic to do that redirect fairly easily. I'm just not sure whether thats really a better solution than what we do now or not.