Well, the redirects I'm currently dumping are linked from nowhere ... except other pages from the same long-gone directories. (googlebot looks around in bewilderment: "###! I swear there were links to this page around here somewhere!") And, of course, sitemaps last seen in 2010.
I do have some redirects that will stay forever, but they're for pages that a human might really try to reach by mistake. For assorted historical reasons I have a fair number of directories that contain pages but no index page, so each of those potential requests gets pointed to the appropriate place. (Never, ever the top-level home page.) No point to changing the structure, because then I'd just be redirecting the ### search engines instead :)
It's like a business putting up a sign saying "We've moved." Eventually they will take down the sign. Doesn't mean they've gone back to the old location; it just means that everyone who might be interested should know by now.