A rule can still "not apply to everyone" and yet not be an exclusionary, "if you don't have this you cannot do this" rule.
Consider: "We are offering a discount on green cars." This does not mean the dealership cannot offer a discount on cars that are not green, nor that the only way to get a discount is to buy a green car. All it means is that you don't have to negotiate for a green car.
Or, since legal stuff is already injected into the thread, consider the idea of an "absolute defense" against a particular alleged act. An absolute defense is something that, if demonstrated in the court, constitutes a guaranteed success for the defendant against a particular charge. There are several examples. Self-defense is one of the most commonly discussed absolute defenses; so long as certain criteria are met (usually regarding the need for force and the proportionality, e.g. striking someone with your car when they're armed with only a knife is probably not proportionate!) Another common one is demonstrating the truth of an allegedly defamatory statement. You cannot defame someone by saying true things about them, so if it is true and can be shown to be true, that's pretty much a slam dunk victory for the defense. Others exist in other areas of law, I'm sure.
Point being: does the existence of the self-defense doctrine thus mean that if you weren't acting in self-defense, you automatically lose? Hell no! It's just one guaranteed successful defense, IF you qualify for it.
That's how class rules should be understood in D&D. They are not, in general, rules of the type "the President has the right to veto laws" (a special dispensation which cannot and will not be allowed to anyone else). Instead, they are generally "if what you said was true, then it cannot be defamatory" type: you can still win against a claim of defamation despite not showing the truth of the statements, it just requires more effort and other factors in play. It is a guaranteed path to success, but not the only path to success.
That's why I keep harping on this. Snarf basically just said, "hey, this principle happens a lot, so that's exactly what's going on here"....but never actually defended why that principle should be applied, rather than the alternative where negotiation is always possible but sometimes unnecessary.
The thing is, it depends entirely on the implementation of the rule.
The example that was brought up was the Actor Feat, that allows you to Mimic other persons voices.
It is worded in a way, that makes it clear that without that feat, you can't mimic the speech of another person.
"- You can mimic through speech of another person ..."
Thr actor feat in that regards only makes sense, if you can't mimic the speech of another person. It even wants a check.
So anybody who took the actor feat would be rightly pissed, if another player could just make a "mimic speech" check now, because that is the main selling point of this feat.
Now, nobody takes the actors feat and it is counterintuitive... because in the case of 5e it bad implementation.
A better implementation would be "You now have advantage on charisma (deception) checks to mimic another person you have heard speak for at least a minute ..." - a solution that was also proposed here in the thread.
Now this rule doesn't make the use of Mimic Speech exclusively by this rule, everybody can still try it. It leaves it open in the Arnesonian Space while the original rule puts it firmly in the Gygaxian Space, closing if off from the Arnesonian Space.
5E does both with rules. You have closed rules - like a halfings nimble ability "you can move through the space of any creature that is a size larger than you". Just by reading that, you know "oh, not being a Halfling means, I can't do that". There is also the general rule in the combat section, saying that you can "only move through a hostiles creatures Space, if it is 2 sizes larger".
Now, those two rules together put the whole thing firmly in the Gygaxian Space - there is no wiggle room, no possibility to move through a hostiles creatures space that is only one size bigger then you unless you have the nimble feature.
A less obvious rule is everything about magic. There is no "negative" general rule that says "You can't do magic" - at least I didn't find one spelled out in the magic section of the 2014 basic rules. But just by having the current spellcasting rules it is clear that a character can't do magic, unless a feature grants him access to spellcasting.
Just by having the spellcasting rules in that form exist, made casting magic got impossible in the Arnesonian Space.
Not every rule makes it impossible to do something in the Arnesonian Space, it is how it is implemented and intended.
For example "You can steal stuff. To do that you need proficiency in sleight of hand - then you make a dexterity (sleight of hand) check would put stealing stuff directly in the Gygaxian Space and shutting it off, exempting every character who is not proficient to do that.
In contrast, a "In order to do magic, a Character needs to make an Intelligence (Arcana)-Check and describe what his magic is supposed to do, the DM decided Based on the desired Effect, the DC" - now Magic can be tried by everybody, a rules written that way leaves the Arnesonian Space open.
Or lets go back to 5e 2014: praying. Unless you are a cleric of a certain Level, you can't pray (and expect anything to happen). Because the only way to pray implemented into the book is the divine intervention feature. So in general, PCs wo try to pray don't get anything from DMs so far I have seen.
And answering prayers for Non-Clerics would rightly upset Cleric Players, because they only get the feature at level 10 and only a 10% chance at that level.
So again, all the God-Player interaction is deep in Gygaxian Space and closed of the Arnesonian Space and limited to High-Level clerics.