It's an interesting question, but from an SEO point of view an old page of content has plenty of power & I wouldn't delete it if possible.
I don't know of an umbraco package that will tell you which pages don't have links - but Google will tell you. If you want to know what links to www.example.com/oldpage.html then type the following into Google:
link:www.example.com/oldpage.html
(https://support.google.com/webmasters/answer/55281?hl=en)
I wouldn't delete old content (unless it is now plainly wrong) I'd keep all the old content & tidy it away:
- Create a folder under home called something like 'archive'
- Move all your dead content there
- Use a 301 redirection package or change urlrewriting.config in case there are any external links to these dead pages as the old urls will now return 404 errors
- Make sure that the sitemap contains links to these new archive pages
You will now have cleaned up the structure, but without losing any SEO value.