#1 is used by a lot of sites, mostly torrents and the like to protect their users and themselves from obvious liability. But I even built one to help a forum that discussed philosophy and politics to help them avoid being invaded by trolls just because they discussed (and linked to) content on far-right sites, etc.

#2 was the one I couldn't think of a great example for, but thought that maybe the BBC were doing this (I have no citable source for this hunch but recall a page discussing programme identifiers and moving all existing URLs to this new structure using redirects).

#3 I agree does not happen in real life. Which is a shame.

#4 Twitter claim to do this https://support.twitter.com//entries/109623 , and I believe Google are doing this.

Edit: Citable source for #3 http://www.bbc.co.uk/blogs/legacy/radiolabs/2007/11/urls.sht... . The BBC use a URL shortener in addition to the equivalent of mod_rewrite to normalise all of their content behind permalink short URLs.

#3 actually happens in reverse: link shortener dies or the shortened link expires, while the actual URL lives on. This way, shorteners are _accelerating_ link rot.

#1 is done by pretty much any site that lets you display "third party" content where said third-party is untrusted. Such as most webmail providers, when loading remote images, and often for links too.

(but there's no reason to do shortening)

1 happens all the time no reason to make it a "short" url though.