1) it is uh difficult to make a rule preventing this. if you're, say, faust, and you like to open each game with 10 paragraphs about the setup then it may be a little bit annoying to not be allowed to use the words of any of the roles.
2) i have seen almost exactly that on MS - somebody made a sentence somewhat obviously outlining the word "tracker" in the middle of a rather large wallpost. appeal to authority
3) the nice thing about this is that it's rather obviously harmful to the game and banning it doesn't really cut off anything somebody would want to do otherwise. if you say "nobody is allowed to make an obviously encrypted post and then later, by some unique algorithm, give means to decrypt it" nobody's gonna be like "drat. i was hoping on jokingly posting some huge string of characters and then jokingly give means of decrypting it in attempt to find scum"
4) see #1. you can't give any means of decrypting it that wouldn't be apparent without any input from you.
5) lalight came up with the clever ruling that the mod doesn't actually have to flip flavor names.
6) "
7) honestly, rolecop should just give results in the scum thread
8) rarely is it the case that there is someone you have non-main thread communication with but you don't want to hide this.
in theory you could start each N0 in the scum thread by saying "let's kill faust. [next post] actually let's kill schadd [next post] actually let's [...]" and then in twilight say "hey, scumpals, let's do the thing i suggested in /m4." and of course the mod can use their discussion to make twilight longer.
the thing about mafia is that it's so easy to break so many of the rules without punishment (e.g. talking by PM or whatever) and most people are operating pretty solidly on a moral compass; if most of your energy is spent figuring out how to properly break the game and use stuff other than discussion in the main thread to win, maybe this isn't the game for you, and indeed people tend to just phase that stuff out because it's not fun. similarly, mod discretion means you can always just observe something in the game and make the judgement call of "this would be detrimental to the fun of the game" and shut it down.
one can also make rules changes to obviate a lot of encryption-based stuff (i think we should consider making it policy that anything easily communicable in a small piece of information, e.g. rolecop result, is just printed in the scum QT by the mod; perhaps, as well, scum should be able to talk in the thread the night after they're lynched)