Buttermilk can always be replaced by the same amount of milk with some lemon juice mixed in (let stand 10min), or white vinegar. Yogurt can also do the trick––basically, you need something acidic.

That said, buttermilk keeps for many weeks in the fridge (and eventually turns to cheese), so it’s a pretty stable dairy product to buy.

I love and hate potlucks, for exactly the reasons you describe above. I grew up pescatarian, so potlucks in the 80s and 90s were a minefield for me. I ate a lot of dessert and starch! I am way less picky now, but dislike a lot of common ingredients (sweet/bell peppers, celery, raw cucumbers), so that presents some challenges. Plus, I’m a bit of a food snob! (I would likely decline that WalMart fruit salad..)

But!! I really love potlucks amongst friends. Even people who aren’t cooking enthusiasts often have a few go-to dishes that are fantastic for such occasions, and I love the implied challenge to bring my best whatever (cake, vegetarian main, green side dish). The jumble of flavors is casual and welcoming.