Quickwit is another project aiming at this sort of stateless search: https://quickwit.io/ . I've been keeping an eye on it for a possible project that would involve wanting to offer search over a lot of data for an application that wouldn't have very many users, such that keeping a big ES box around all the time would be needlessly expensive; I had been looking for a search situation where my only always-on costs were for storage, and I could pay for compute as needed (and ideally also separate paying for indexing compute, which doesn't happen often in this application, from search compute).

Interesting that ES might also end up with this kind of an offering.