The Codex does not document the $taxonomy argument. Without digging into it, my assumption would be that maybe it originally was not one of the arguments and no one ever bothered to change the Codex.
The Codex, while still a tremendous resource of information, is (1) user generated/curated content and can be prone to mistakes, and (2) is no longer the “official” documentation of WP core functions, hooks, and APIs. That status belongs to https://developer.wordpress.org/.
On the developer site, the $taxonomy argument IS documented for get_previous_post(). Since that data literally comes from the WP core inline documentation, that right there tells you that the Developer Docs are correct and the Codex is wrong.
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