Unfortunately that limitation is hard-coded, but it is hard-coded to the dimension's "sea level", not necessarily Y=63. In other words, to get around it, you could edit the world data (or worldgen data pack? not sure) to set sea level e.g. to Y=0. Do note that this has other effects, such as also getting phantom spawns down to whatever Y-level you set it to, or glow squid only spawning well below that level.