I don’t think you understand what “bright line rule” means. Wikipedia has a pretty good definition (though it references the US Constitution, it applies to just about any usage of the term). It means that the rule must be strictly enforced as written. So it depends on how the rule is written.
When you say “the rule is no racial slurs”, is that a rule you’re proposing? Because that’s not the rule in the BBQ Pit currently. The rule currently is:
If you say hateful and/or racist things, you may get warned or banned. Some slurs are likely to be viewed as hate speech when used as insults, some aren’t. No, we aren’t going to give you a list. Our goal is not to restrict any and all speech which could be viewed by someone as offensive, but simply keep a modicum of decency, even here.
So you are proposing a new rule simply called “no racial slurs” with no other definitions? That’s vague enough that it might be meaningless as a “bright line” rule, because there is no definition of what a racial slur is, or what is considered a slur, or what makes it racial, or even what “no” means. No discussing slurs? No using slurs? No oblique reference to a slur without mentioning what it is? It’s not much more useful than “don’t be a jerk” if you intended to make that the “bright line”.
That’s a meaningless statement. Again, it depends on how exactly you write the rule. Based on what you are proposing those could absolutely be violations.
I’ve dealt with bright line rules before, the easiest one to bring to mind from Wikipedia is the “Three Revert Rule”. You cannot make three reverts (undoing someone else’s edit to an article) in a row without some kind of intervening edit between them. They could be seconds apart or days apart. Two reverts is okay. But once you make the 3rd, administrators are supposed to block you. This is ostensibly to prevent an edit war from breaking out (where two or more people are undoing each others’ edits without discussion). But I’ve seen all kinds of attempts to game the rule. I’ve blocked many people while enforcing that over the years, and I’ve sometimes disliked doing it; I have seen where the person being blocked is clearly the one in the right, but I have to block them based on a technicality because it’s the rule.
Another one is “no legal threats”. If you threaten to sue another editor on Wikipedia or the project itself you get blocked indefinitely unless you retract the threat. That is to prevent what is called a “chilling effect”. I have even blocked a celebrity in one case who was threatening legal action. I’ve found that to be a good bright line rule in my experience. So sometimes they work.
The way that this board is, I don’t see how bright line rules will improve the place. I think the moderators do well enough using their judgement, and the rules that are in place leave enough room for interpretation that they don’t need to be defined more. Moderators do occasionally get things wrong but there is nothing you can do to prevent that because they are human. (As far as we know.)