What is the expression for rules that are typically flouted?

Solution 1:

You are probably thinking of more honored in the breach than the observance, which can be found on phrases.org and on wiktionary under "honor in the breach":

(idiomatic) (of a rule, law, or policy) to demonstrate by breaking or breaching  (wiktionary)

This is usually thought to mean a rule which is more often broken than observed. The context of the play [Hamlet] shows the real meaning as 'it is more honorable to breach than to observe'. (phrases.org)

Personally I find phrases.org's "real meaning" to be poorly phrased; it should say something like "the original intent of the phrase was to point out that it is more honorable to break certain rules than to obey them." The quote is from Hamlet, specifically when Hamlet was observing that it was customary for his stepfather to get all liquored up and engage in shenanigans. He then says

...But to my mind, though I am native here
And to the manner born, it is a custom
More honour'd in the breach than the observance.

Solution 2:

A rule that has little or no practical use and is usually ignored is a dead letter. The other American English meaning, "an unclaimed item of mail," is uncommon in British English. http://www.oxforddictionaries.com/definition/english/dead-letter

Solution 3:

Rules that are not normally followed, or 'observed', are called 'unobserved'.

I can't find any reference that defines the phrase "unobserved rule", but searching for "unobserved rule" returns plenty of example usages.

'Unobserved' doesn't always get its own entry in a dictionary, as it's just a standard modification of 'observed'. It's negating this meaning of 'observe' (from M-W):

1: to conform one's action or practice to (as a law, rite, or condition) : comply with

So an "unobserved rule" is one that was not or is not followed.