Sodium now has support for the Fabric Rendering API built in, so you no longer need Indium.
Indium also isn't compatible with Sodium 0.6.0+, so don't use it :)
Indium is an addon for the rendering optimisation mod Sodium, providing support for the Fabric Rendering API. The Fabric Rendering API is required for many mods that use advanced rendering effects, and is currently not supported by Sodium directly. Indium is based upon the reference implementation Indigo, which is part of Fabric API with source available here. (licensed Apache 2.0)
Any mod that uses the Fabric Rendering API will require Indium when used with Sodium. These include: Campanion, Bits and Chisels, LambdaBetterGrass, Continuity, Packages, and many more. Some of these mods may function without an implementation of the Fabric Rendering API, but have broken textures and models.
Indium's impact on performance should be negligible, however mods that use the Fabric Rendering API could themselves impact performance. Indium will not provide a performance benefit over using only Sodium.
No, Indium is an addon mod for Sodium - you must use both mods together to get Fabric Rendering API support with Sodium's rendering optimisations.
No, Indigo (not Indium) is provided as part of Fabric API as the reference implementation of the Fabric Rendering API. Indigo disables itself when Sodium is installed.
Yes. To use Sodium with mods that require the Fabric Rendering API, you need both Fabric API to provide the rendering API and Indium to implement the rendering API.
Fabric Rendering API support is not currently a priority for upstream Sodium, and will not be explored until Sodium is in a more stable state.
Sodium 0.2.0 or newer is required for all versions of Indium. I support the latest stable versions of Sodium for 1.18 and 1.19. Iris may not always be compatible with the latest release or build of Indium - please ask the Iris support channels if you have issues.
Releases of Indium are available from Modrinth and CurseForge, as well as Github Releases.