"languages used must pre-date the challenge"
This has been codified in an old meta question asking "What programming language should we consider for the code-golf solution?, where the (by far) top-voted answer simply states, in its entirety:
I think that as long as a compiler was released before the question was asked, it is a valid language. This removes stupid answers such as "I created language X with this command Y that solves this in one character".
No such rule is present in the tag wiki (it probably should be added), but another clause is already present:
"it's an old joke"
This originates in the code-golf tag wiki, where the heading "Things to note" contains this point:
- The language you use should not be invented just for solving this particular code golf. It's been done, trust us, and the joke is old hat.
Does this rule actually exist [in the Help Center]?
The only page that is site-specific in the help center is /on-topic. It has a heading for answering and even a heading for code golf in particular, but currently does not specify the language choice criteria anywhere. The points for golfing just include a shortened version of the tag wiki and basically say "strip whitespace and use one-character identifiers * adding an ungolfed version is nice * be sure to include the character count".