I do not think an advice like this is applicable to any programming language out there. For example, I find it hard to apply to languages which have long history and change a lot over time, like Python or Java (or C++ as mentioned in other comments). I can easily imagine parts of the standard library being written long time ago, with many modern features not available back then.

For example, I do not think it is possible to learn modern Java by reading Java Collections code. You won't see Optional<T> being used there. Instead, your takeaway might be that it is perfectly fine to return nulls all over the place.