Ever since 21w37a, support for custom structures with worldgen has been removed. There was no mention of it in the changelog, and no official word on it being removed, that I know of, which leads me to believe this may be a bug. Editing an existing structure file (such as a village's template pool) does not do anything. Meaning that structures are currently hard coded into generation and cannot be modified in any way by either changing/editing the "template_pool", or the  "configured_structure_feature" files.

I have attached two biome json files below, comparing the difference. As you can see, in the 1.17.1 one biome, there is a space for structures to be listed, called the "starts" field, while in the 1.18 biome, you can see the structures list has been removed.

I have also attached two data packs. One is the 1.17.1 version with custom structures being able to use it, and the other one is the 1.18 version that is identical but there is no support for custom structures, making it pointless.