The smaller the userbase, the more likely the code is to remain in service forever. The oldest code I've written is http://www3.amherst.edu/~scrutiny/about.php, which has gotten a facelift since I wrote it in 2004, but otherwise seems to have the same functionality and code backing it.

Pretty much everything professional is gone...hell, the only employer of mine that is still in business is Google. When I left them in 2014, about 3% of the code I'd written for them was still in production, and following the rule above, it's silly stuff that nobody ever sees, like https://www.google.com/search?q=deubogpiegpj&tbs=qdr:h (that's the no-results page when a tool is selected).