The most common and most likely answer to this crossword clue is the 9 letter word UNWRITTEN
I've seen this crossword clue in The Guardian
U
N
W
R
I
T
T
E
N
Buddy explains!
The straight part
We know that "Such a rule never makes it into the law books" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case such a rule never makes it into the law books = unwritten.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does UNWRITTEN mean?
UNWRITTEN verb- To erase; to revert to a state where (something) was never written.
- To nullify.
- To deconstruct.
- (computer engineering) To revert to a known state in so that new data can be written.
- Not written.
- Oral or otherwise communicated without writing.
- Implicit or understood but not formally articulated.
- Containing no writing; blank.