I checked on this again. There was an old Search API that has been deprecated. It was part of a package of stuff including Classification stuff, etc.
But I can’t find anything (in spite of that GPT conversation I had) that says that file_search is implemented on that deprecated Search API. So I don’t want to be guilty of spreading rumors.
Perhaps @jim is right and Assistants is alive and well and part of gpt-5. Let’s hope so.