We’re considering how we can express “directory private/public” exports in Notion’s codebase. One idea is to consider file names starting with _ as “private” eg smyFeature/_internalClass.ts could be imported by other files in myFeature/ but not by anything in yourFeature/. But doing file level granularity would encourage barrel modules without the _ to re-export stuff publicly, eg myFeature/public.ts.
These numbers show a good reason to go with a different strategy that allows per-export public/private instead, so consumers only pay for what they actually get!