I still use some old, unsupported plugins and occasionally recommend them.
Old plugins are not removed because:
* They may still work (without any issues at all!)
* Even if they may have issues with newer WP versions and plugin conflicts,
they may still work on older installations. Therefore they are useful.
Plugins can be removed:
* They have unpatched security issues
* Plugin author is unresponsive (unreachable by sending them email, this is done by plugins team)
* Plugin is otherwise violating WordPress.org plugins guidelines.