The MySQL Lifecycle Policy (http://www.mysql.com/company/legal/lifecycle/) determines which versions are actively supported, and for which platforms such support applies.
The basic principle is that old versions are supported for a quite long, but definitely limited period, once they have been replaced by a newer GA version. For example, since the introduction of this policy, MySQL 3.23 and 4.0 have been retired.
The policy contains also provisions for a different kind of End of Life dismissal. When support for certain platforms has been discontinued by their vendors, of the platform is not widely used, MySQL reserves the right to stop building binaries and testing code on such obsolete platforms.
The reason is simple. While hardware can be bought and stored, time is a commodity in short supply, and there is only a given amount of time