Svelte knows about Typescript natively (you can even use it in the template! Not possible back then), CSS became more powerful and Tailwind appeared, making less/scss/typescript preprocessors not needed anymore.
And many preprocessors are better served as Vite plugins anyways.
Some background: svelte.config.js was originally created so the VS Code extension knows about the compiler options and - more crucially - the preprocessors.
They were very common back then because you e.g. had to use one to get Typescript support in Svelte. Nowadays
So we rethought our assumptions and realized we don't need the config file anymore.
We can use Vite's resolveConfig method to do the heavy lifting of transforming a file path into a vite config object, then check if our plugin is in there to get the options from it.