The damage_type 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-260446, MC-251318 and MC-187938.

How to reproduce

1. Download the attached data pack
2. Add the data pack to a world and run /reload
3. Run /summon arrow ~ ~3 ~
   → Notice that nothing changed.
4. Leave the world and re-join the world.
   → Notice the warning "Worlds using Experimental Settings are not supported".
5. Click "I know what I'm doing!"
6. Run /summon arrow ~ ~3 ~
   → Notice the burning effect sound. Only now the damage type modification is loaded and working.