🟢 Shouldn't this be "on"?

That rule is mostly useful for applications or larger libraries with plenty of optional imports. What would be the benefit of having indirect export structure in a library with just a few exports?

I would simply aim for consistency, and never use barrel files (no matter the size of the project). Or are there any scenarios where we still want to use them?

note that if you don't have a barrel file in a library, you can no longer have imports like

import { Client, RequestError, EMPTY_SCHEMA } from 'my-library'

unless they all come from the same file. For me this also brings worse DX, or clutters code structure of the library.

So for libraries where tree-shaking is not important I'd still go with barrel files. maybe we need separate biome preset for them

Fair enough. We need to find the right balance with this.