Postgres is a weird one. The first link probably answers the query, just click the latest version (or your version) once you are there.

The problem is probably so many systems run old versions, so the results skew.

Hey, at least it's not ads?

You didn't include a version in your query. You also could try using quotes, though this specific entry may not be helped by it (e.g. "in operator"). For most things, you can click a link with the older version and somewhere there is typically a dropdown or something to change the version and, if not, you'll at least know which section/etc. it is in in the new documentation.

If you don't include a version, it's probably going to pull up questions/answers that it finds most match in general and maybe people just aren't asking that question for your version.

I think there's a lot to hate about modern search results, but I also think there's some opportunity to search better. I do miss the days when AND, OR, and NOT operators actually worked all the time and as expected.

Skill issue. Old version docs tend to offer you a redirect to more recent docs, and even then something sintactic like an "IN" operator is unlikely to change in form or structure between versions of a database engine.