The Meaning of Life, the Universe, and Everything.
Brown dye is a rarest dye in the game. To get it you should find Jungles first. And it is very hard to find Jungles! Then you collect cacao beans, and carry them back to your base (at the opposite end of the World). So it is very challenging for player to get brown dye, and it makes game really interesting!
But natural spawn of Brown sheeps just kill that. You can get Brown sheep at first minutes of the game, breed them, and you have brown wool any time you want. You can craft carpet, banners, etc. You don`t need brown dye anymore (at least in survival singleplayer game).
So, forbid natural spawn of brown sheeps, please. Let player find cacao beans first, make brown dye, dye sheep, breed it, sell cacao beans (or brown dye) to any villager, AND THEN let them spawn in those villages.
Player should be able to obtain brown dye from wonder trader, though.
The only items brown sheep give you access to are brown wool, brown carpet, and brown beds. As mentioned by erictom333 and AMPPL50, there are plenty of other items that you still need to find a jungle to craft.
This seems like a pointless removal of a feature. No Support.