While Realms does not support experimental features such as custom worldgen, creating a dimension which only consists of a single vanilla biome is accepted by realms and allows you to visit them in the realm itself. When downloading the world, the changes in these dimensions are saved, but the way Realms verifies files when uploading a world will omit the chunks from these dimensions, essentially clearing them when re-uploading a world
Note that this is not the same "bug" that prevents thing like the Caves and Cliffs datapack from working on Realms, as that is a deliberate design choice. The fact that the system does not try to upload the regions from the custom dimensions makes me believe this is a design oversight, as it should either properly upload the region files or not allow these dimensions to be accessible on Realms
The attached datapack consists of dimensions which are only a single biome. These work with realms and you can visit them, but they are reset upon re-uploading the world