Chaosmancer
Legend
Fantasy plants. For the same reasons a Triton may just be able to dive deep into the ocean, halflings may just be able to grow the plants they need for delicious meals.
Trade exists to address scarcity of resources. And the lore seems to suggest that halflings don't need much. Add in fantasy, and a group of people can address scarcity internally, reducing or eliminating the need for trade.
Not gonna say that it all holds together, but I see no reason that this part shouldn't.
Sure, maybe fantasy plants.
But, think this through. Nothing about halflings suggests that they have plant magic. So, these plants would be something anyone can grow. There is nothing in the lore about them having access to crops no one else can grow.
So, anyone could grow all of these fantasy plants in their farms. So, since humans and halflings tend to live so close together, humans would be growing those plants.
So, Humans no longer need to trade for spices... so what are they trading? Because we have plenty of trade cities in DnD. But if they don't need salt, pepper, or other spices what is driving trade? Those were the three biggest drivers of trade in history, and we've waved them away. Is everyone trading in steel, swords and armor? Why? Perhaps all trade is in stone? Why?
Maybe we could find enough things to keep trade going in DnD worlds, but this change deeply impacts any city famous for trade, any famous merchant, the idea of shipping or caravans that you escort as guards. It cuts deep into our assumptions about DnD.