I know that the parts of the system can be omitted, but the core books are a communally referenced standardized rule set. Most DnD games use some combination of most DnD official rules, so it's fair to revert to "default DnD expectations" until you reach a specific case that has to pull parts out.... and the alignment options are in the book.
Again, this may be a shock to you, but just because something is in a book, even a core book, doesn’t mean it gets used in a campaign.
And I will reiterate again, whatever standard you are using for “fun-impacting” decisions, you might want to apply for “make the DM work more and do things she doesn’t want to do” decisions. Because you seem to treat those cavalierly (and not in the sense of the class).
Fun impacting decisions is simple- does the player want to do something that they find to be fun, and would this decision impact it? As for the DM work, that's as flexible as the DM wants it to be. Even if you're serious on world building, there's lore to take inspiration from, the narrative device of "lost history" or whatever else you want to do. A lazy mind is capable of great invention. Even if you just take "Gith" and make it an occasional type of Elf, problem solved. You put in what you want. I can respect someone who wants to go full force on the world building, but I don't think that is the standard. This may just be me, but I'd think that a world builder wouldn't have a big problem doing more of what they already like. The situation for this being a problem seems kind of specific- a DM would want to hard-build an element-restricted setting, and not have talked with the players enough before hand to get them on board with the restriction.
Like I've said, restrictions for a "vision" is fine, and this would be that case. We can't say that every option restriction is done under those circumstances, especially if "Monster races shouldn't be playable" is a semi-prevalent topic separate from setting.