Can I search HN, sort by date and hide all the hits which have never been shown in the top 30? Occasionally I want to find something I've seen among the titles on the HN index page some months ago.
Yes you should use https://hn.algolia.com !
I do. But how do I filter never-been-among-the-top-30 titles out while keeping the list sorted by date rather than by rating?
You can only sort by popularity, which maps to 'score' in the data structure that the API sorts. You can see that here: https://github.com/HackerNews/API
The highest position on front page is not, AFAICT stored in any way and is not searchable.