I was trying to implement PluralRules for Boa and I saw this API wasn't implemented.
I'll try to contribute this :)

Cool! A few tips to get you started:

1. We need to start consuming pluralRanges.json from CLDR-JSON:
   1. Add it here:

      icu4x/tools/testdata-scripts/globs.rs.data

      Line 32 in 687d0ff

      "cldr-core/supplemental/plurals.json",

   2. Then run cargo make download-repo-sources
2. Now you can add a data struct for the plural ranges into icu_plurals::provider. This is the most interesting step because you need to design a data structure that is correct, compact, and efficient. It is usually not a good idea to directly copy over the JSON structure. Note that we'll end up with one data hunk per locale, which is different than pluralRanges.json is structured.
3. Once that is ready, add a transformer for the data in datagen/src/transform/cldr/plurals, then add the data key to plurals/data/config.json + datagen/src/registry.rs, then you can run cargo make testdata and cargo make bakeddata components/plurals to see it all transform before your eyes
4. Finally, you can build the actual API for the feature. I'd suggest a new struct called PluralRanges in the icu_plurals crate.

Each of these steps can be done as a separate PR if you prefer.

This process is all documented in writing_a_data_struct_md.