This was a long time ago, but my oldest was deferred from EA from Caltech. They sent out acceptances via priority mail and rejections regular mail. It took a week for the rejection to come. By then of course we were pretty sure that’s what it would be, but it was disheartening coming to CC and seeing that the acceptances had arrived and having no idea whether the envelope had just been lost and at what point one should call their admissions office.
Carnegie Mellon did acceptances by waves - I think it was related to the fact that each school does their own decisions. Thankfully my kid was in a relatively early wave, though not the earliest.