This post is not about legal systems, but an (imperfect?) analogy about game design. Let's have some definitions for this purpose (
source):
Civil law systems have their origin in the Roman legal tradition. Civil systems vary widely, both in procedure and substantive law, so conducting research on a particular nation's civil law system should include looking at that nation's specific system of law, but they do have some trademark characteristics. Nations with civil law systems have comprehensive, frequently updated legal codes. Most importantly, case law is a secondary source in these jurisdictions. France and Germany are two examples of countries with a civil law system.
Common law systems, while they often have statutes, rely more on precedent, judicial decisions that have already been made. Common law systems are adversarial, rather than investigatory, with the judge moderating between two opposing parties. The legal system in the United States is a common law system (with the exception of Louisiana, which has a mix of civil and common law).
The idea of civil law is basically "think of how the law should apply in abstract, pass a law and then enforce it". In game design, this would be thinking of a cool feat, comparing it to existing feats, and adding it to your game before play-testing it.
Common law would be the opposite approach: play the game until you reach an impasse. Resolve the impasse. If the problem arises again, you already have an answer, which might be enough or might be superseded.
There is also this:
Customary law systems are based on patterns of behavior (or customs) that have come to be accepted as legal requirements or rules of conduct within a particular country. The laws of customary legal systems [...] are often dispensed by elders, passed down through generations. [...] Oftentimes, customary law practices can be found in mixed legal system jurisdictions, where they've combined with civil or common law.
In RPG-land, these are the rules we used because they were always like this. Falling damage ("d6 per 10 feet") is a good example. It is not particularly realistic or deliberate, and it's basically unchanged from OD&D to 5th edition because nobody cared to change it.
One could say "corporate" D&D is civil law (created by designers for users) while indie/OSR (and probably the ORIGINAL D&D) games are common law (created by users for users). "Grognards" would prefer customary laws (created by Gygax for the good of humanity). I jest, I jest.
In reality, everything is a mix of all systems. Many OSR writers use old rules without question, and WotC often use feedback from actual players before publishing a new class or feat.
But the "civil law" mentality in corporate D&D might explain why some 5e homebrewers, unlike, say, OSR authors, seem so cautious about changing the fundaments of the system; they do not seem themselves as legislators, only judges.
On the other hand, there are plenty of people dedicated to create innumerable rules and classes... that their players don't need. They are deep in theory-crafting and balancing, but not really into playing D&D. Here are two pieces of advice: first, do not fix what is not broken, unless you really think you can make the game cooler. Second,
nobody wants your house rules... or do they?
This mentality also shows in official modules (and also some unofficial ones). Some books seem like they were written to be consumed, while others are obviously written to be played.
Curse of Strahd, one of my favorite 5e books, is a good example; it feels somewhat "customary" ("we must do that because it is in the original module") and somewhat "civil law" ("this will certainly look cool in the book"), but not "common law" at all ("this didn't work in practice, let1's try something else"). I get the same feeling with the "infernal machines" in Avernus (this LOOKS really cool doesn't it?).
Here is a more specific example: you come up with a rule that requires rolling a lot of dice. You make some calculations and find out that this means +0.7 damage in average, exactly right for your purposes. This is "civil law". If you are playing the game and realized adding half a dozen dice rolls per round for a 0.7 is a hassle, this is "common law".
Again, this is rough generalization. There is plenty of "civil law" mentality even in indie systems - i.e., the idea that the game designer knows the table's problems better than the GM. And, to be honest - sometimes they do. They might have spent more time thinking about this stuff, at least as far as their own games are concerned. However, they do not know what you and your friends enjoy... or hate.
My own books always START with the "common law" approach (i.e., I want to write books I already NEED), but add some "civil law" as I write, because I want the whole thing to be a coherent whole.
Anyway. Play the game long enough, and you'll find that it becomes harder and harder to play with rules that are not your own and not specifically suited for your table.
