(hmm this is somewhat off-topic, but will let others decide on it)
Better answer
Although it is hard to track official announcement, as @darmb pointed out in the comments there are indications that the official documentation effort is drifting away from the codex in favor of the developer site, and the “make” blogs/sites. So in a way, while not officially announced (as far as it is possible to see), the codex should be treated as if it was deprecated.
References:
- “…with the ultimate goal being to eventually shut down the Codex.” (link)
- “The Codex is deprecated, and is slated to go away at some point. Thus, the Theme Review page on the Codex is no longer official, or maintained.” (link)
Original answer
The codex is a wiki style site. It being up to date requires some volunteers to actually go and edit it. Usually with new code you will get something there, although quality might not be the greatest, but it may suffer from “bitrot” on older topics with minor changes.
The developer site is a phpdoc style site auto generated from the phpdoc style comments accompanying function declarations and hook usage , therefor it is in sync with the actual code (and you can actually see the code there).
Pain point of codex – it may diverge from actual description if no one cares about some API. Strong point it is human generated for humans and in theory easier to edit.
Pain point of the developer site -it is only for latest version and depends on the quality of comments in the code, something that on avarage may change only every 4 months. Strong point – it shows you the code it self, being a replacement for using the text editor to search in the code.
I think that misleading information is worse then no information and therefore I try to avoid the codex, but no, it is not deprecated, and if you find a gap there you are welcome to fill it 🙂