I haven't found any height maps for Atlantis, Last Cave, Ascension Island, or Sky's Rim so if you by any chance have them please message me so I can get them imported into 1.15.
From what I remember (correct me if what I'm saying is wrong), the height map for Ascension is identical to that of the old Bonfire, and the height map for Sky's Rim is defaultTerrain.png.
Anyway, thanks for doing this. I (and, going by the thread, and the general reactions to the June update, a lot of people) was really missing the old maps. Especially Bonfire.