The trim_pattern and trim_material JSON files are considered experimental, just like the worldgen files. This in contrast to the fact that they're clearly documented in the changelogs, and the changelog did not mention that they are experimental. As a side issue, this also means /reload-ing does not work for them.

Relates to MC-251318 and MC-187938.

How to reproduce

1. Download the attached data pack and resource packs (from https://github.com/misode/trim-examples/)
2. Add the data pack to a world and run /reload
   → Notice that nothing changed.
3. Leave the world and re-join the world.
   → Notice the warning "Worlds using Experimental Settings are not supported".
4. Click "I know what I'm doing!"
   → Notice that only now the trim pattern is loaded and working.