This little-read and little-voted-upon post deserves a LOT more attention than it received
Ignoring our 3rd-party/commercial world rule for the sake of this example,1 there's a clear line between the following:
Off-Topic: Questions asking how to develop game mechanics or player interfaces as found, for example, in the Dungeons & Dragons Player's Handbook.
On-Topic: Questions about world rules in the context of game mechanics such as TSR's Greyhawk.
From a practical standpoint, both of those documents contain rules. The difference is that one contains rules that explain how someone plays the game and the other contains the rules for interacting with the world.
An argument could be made, what's the difference?
The difference is that game mechanics involve rolling dice, referencing charts, and can be applied to any world. A perfect example is the Unreal Engine, which can be used to implement pretty much any world, but does not inherently impose restrictions on the expression of any world.
We're not here to help people build their version of the Unreal Engine. We're here to help people create worlds they may later implement using the Unreal Engine.
And that's the difference.
Had I seen the linked question while it was still open I would have voted to close it. It doesn't matter to me how integrated the process by which players play the game is integrated into the world. If we open that particular pandora's box, then we'll be answering questions about how best to explain information that was once found in game play manuals to players via the world in the game.
That's not what we do.
In fact, it's not worldbuilding. It's user experience building.
No matter how integrated the process of playing the game the reality is that the two can always be completely separated. In fact, Nepene's linked question could have been asked by replacing the example of the fireball with a laser pistol, or a sword, or Jungian Psychiatry, or any other example, because the question was about how players interact with the game and not the world. The rules may be intrinsic to how the game is played, but they're not intrinsic to the world at all. Once the question was answered, it could be universally applied to any game Nepene chose to create because it's independent of the world.
Do we have a precedent?
Kinda yes: Does the Stack wish to allow questions concerning the profession of worldbuilding?
And kinda no: How would you make money by "worldbuilding"?
If we don't answer questions about worldbuilding as a career, then I believe it's quite a stretch to allow game mechanics no matter how tightly bound to the world's design. If we were to allow questions about game mechanics, I believe (very strongly) that it would first require convincing the Mods that twelve votes constitutes community consensus toward an adjustment to the Help Center pages permitting career questions.
But until that and a similar modification is made on behalf of game mechanics, the simplest answer is that they just don't meet the Help Center rules.
Questions about game mechanics are off-topic.
However, if there isn't another stack focused on developing game mechanics, that seems like it would be a shoo-in for Area 51.
1 And this is important because I'm going to advocate that we are NOT the place to develop the game mechanics. In other words, if you create your own game mechanics and then want help building a world around those mechanics, we're glad to help. But developing the game mechanics themselves... not our job.