When i pass from 1.15 to 1.14 all the builds get deleted in the overworld, the nether and the end. It's like if you started the world in the same seed, because everything gets reseted. The only difference is that you keep your inventory and the ender chest's inventory. This happens at least in survival and creative. The first three photos are from the 1.15.2 world, the other three photos are from the same world but on 1.14.2.
Sorry if there is something wrong with the spelling, english isn't my first language.
Downgrading is not supported.