Well, I guess Proxima b, is just recently in the habitable-zone; in the past, Proxima-Centauri was hotter and the HZ was further away; the implications of this, if Proxima b wasn't a migrating-planet, that the atmosphere was either blown away or, it resembles Venus. It's not a game-stopper for the world being remotely habitable: Earth, for the first 500 million years or so, had an atmosphere like Venus. So, depending on whether it's eleven-and-a-half day rotation is enough for produce a magnetic-field and, how much water the planet, initially had could determine if it has any water, or an atmosphere at all on it.