The idea behind -next is to rewrite the entire library in TypeScript to make use of type safety and other features during development. We will still release it as a JavaScript library in the npm registry.
Being a full rewrite this will be a bit of a larger undergoing, so we encapsulate it in its own repository. Discord.js version 13 is in its finishing strides over at https://github.com/discordjs/discord.js . We do currently not have a roadmap or estimate on a release date for the rewrite under -next.
See Contributing Guide.