Fully agree on search being one of OneNote's previously strongest features which is tanking fast. I also use it dozens of times daily - I have over 20 separate notebooks (each with many sections, the larger notebooks have sections organized into section groups, and each section with many pages) open at all times, with information that I may need to access at any time due to the nature of my work (I work with a vast array of IT systems spanning many different areas of expertise). So search for me is absolutely critical. And what they've done to it lately has totally butchered it.
To piggyback on your point about quoted terms not working properly, apparently that's gotten even worse in v2602 to where quoting a term no longer works at all to return exact matches only. In other words, from my understanding, quotes around terms in the search bar basically have no effect anymore (this is per another comment on this thread). Can't wait π