Drupal has been around for quite a while now, going all the way back to its initial release in January of 2001. Since then Drupal has gone through many different changes (now at the cusp of version 8).
I almost hate to admit it, but we are supporting Drupal installations back to Drupal 4.x. Understand that these websites aren't really "Drupal" any more. Without any type of updates from the community they're effectively just a PHP-based website running a CMS.
My Drupal 5 site is working just fine. I just need to have some basic tasks done every once in a while.
Who would support an old version of Drupal?
Since these websites do not get any security updates from the community, Drupal 5 websites (and earlier) are at a high risk of getting hacked. If your website deals with financial or proprietary data, and is running in Drupal 5, 4 or earlier, you should seriously consider upgrading as soon as possible as it is incredibly vulnerable.
Besides keeping these websites operating, we have also transferred these them to newer, related and supported technologies.