From what I can observer, at the time of the fork from TVT Optional Boss redirected to this page. I propose it be made into a supertrope, on the basis that superbosses are not remotely the only type of optional boss that can be passed up in a given playthrough of a game - there are also games with different plots routes, as well as story modes influenced by certain actions during gameplay (up to and including "karma systems") that can pit you against specific bosses and block off others depending on those actions (Undertale being an easy frame of reference for this).
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