Maybe you don’t want to buy buttermilk for a recipe that calls for just half a cup of it, or maybe you’ve already started cooking and just realized you need buttermilk and don’t have any. Nothing matches the pure taste of buttermilk exactly, and if you really want to taste that flavor—if you’re making dip perhaps—you should try and stick with the real thing. But if you’re baking or making pancakes, don’t worry about using a substitute.
Source: The Best Buttermilk Substitutes
My five – they forgot milk + lemon juice. Someone claimed it was vegan… but it is vegetarian, depending on your practice.