It's not that it's a bad idea in itself--it's just that people tend to frequently overlook categories anyway.
(To be fair, some of them aren't really intuitive, such as the fact that a CentOS server question belongs in Linux Chat, because it's not actually Fedora.)
So, I think it would just wind up causing some extra work for the staff, moving things around. As Ilja said, I think it's fine for people to post about their legacy releases, and of course, always best to post the version in the thread title.
For better or worse, however, most of the time, people posting about older releases are simply advised to upgrade.