Actually, if you are using Time Machine for your backups, those backups do eventually get deleted and replaced with newer ones. Eventually, the backup with your single remaining copy of something important could get deleted. In fact over enough time, it WILL get deleted. Time Machine keeps sort of a moving window of backups in time, so eventually the oldest ones get discarded to make room for newer ones.
If you just copied a file to an external drive to "back it up," then that copy stays there until otherwise deleted by you.
Other software may or may not delete your backed up copy. Cloning programs will delete the file on the backup if it was deleted on your original hard drive.