Both cases are no longer necessary as everything since 1.13 is now a distinct item and no longer a shared one with the metadata being used to distinguish between different things. Which was the case in 1.12 and earlier for parts and materials to save on the limited number of available item ids for all mods.

IDefinitions.parts() might be kept, as they are still a special case. However IDefinitions.materials() are really just a bunch of items with no special functionality.