Really? TBH I've never seen a single complaint anywhere about the old location of the search bar or the fact that a tiny sliver of horizontal screen space was taken up by the containing area. Furthermore, the real kicker is if you use horizontal tabs (which I do), that strip that used to contain the search bar is still there anyway (because it has to contain the horizontal tabs) - so in this case the "reclaimed space" is literally 0. The only way this "reclaims" any space is if you use vertical tabs, and even then - how much screen space did you really get back? A tiny amount.
However, the impact on workflow is significant. The tradeoff is the search box now overlaps a significant portion of long titles, as well as page content when expanded. And for right-to-left languages, it likely just made the product almost unusable. For reclaiming a tiny horizontal strip of screen space at best, or no gain whatsoever at worst. That's a terrible trade-off for user experience; this was not well thought-out at all. I am happy they are looking for ways to improve OneNote and are collecting feedback - that's great - but please, MS, think through these changes and test more thoroughly before rolling them out. They affect literally millions of users who then often have to wait months to get something fixed if it's overlooked, with no option to switch to the old UI in the meantime.