My practice and what I recommend doing when coding plugins is not to bother about backward compatibility, to always code for the latest stable release available or against the trunk version, and not to support plugins coded for old, deprecated and insecure versions of WordPress.
This said, there can be situations where you need to backport something, or to make sure it will work with both older and newer versions of WordPress. If you happen to be in a situation like this, good news fella: I've just updated my handy WordPress Functions Implementation History Tool, which lets you know when a particular function was introduced in WP (or removed from WP if applicable). The tool comes with a full list of all 3920 functions that existed through WordPress' history in various formats. Enjoy!
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