Preheat oven to 325ºF. Butter a 13x9x2" baking dish.
Arrange bread cubes in a single layer on 2 rimmed baking sheets. Toast, stirring occasionally, until dry and crisp, 25–30 minutes. Let cool, then transfer to a very large bowl.
Meanwhile, bring cider to a boil in a large wide pot and cook until reduced to 1 1/2 cups, about 40 minutes. Remove from heat and set aside.
Bring raisins, vinegar, and 2 Tbsp. water just to a simmer in a small skillet over medium heat to warm through. Transfer to a small bowl; let cool, then drain.
Cook bacon in a skillet over medium-high heat, stirring occasionally, until brown and crisp. Using a slotted spoon, transfer to paper towels to drain (do not wipe out skillet).
Heat 1/4 cup butter in same skillet over medium-high until melted. Add chopped celery stalks, apples, shallots, garlic, thyme, salt, and pepper and cook, stirring occasionally, until tender, 10–12 minutes.
Add celery mixture, celery leaves, raisins, bacon, and parsley to bread cubes and toss to combine. Mix in reserved cider and 1 1/4 cups broth.
Beat eggs in a small bowl, then stir into stuffing; add more broth if stuffing is dry. Transfer to prepared baking dish. Bake stuffing until cooked through and golden brown on top, about 1 hour.
