Hello Tony He (or anyone at MS who is listening) - we have not heard from you in quite a while it seems. If you are truly collecting feedback from customers, it would be nice to see a response on all the feedback you've received here. It's interesting you publish an article telling us about the changes and how beneficial they are, and when everybody replies back to tell you that they are not beneficial and in fact have negative impact on the vast majority of workflows, all of the sudden there's no response. To me that feels more like, MS is collecting feedback when it is convenient for them, but not when it isn't (i.e. when it would necessitate them to roll back a change they obviously wanted to push for some reason, even though as others have pointed out, literally nobody asked for it).
It's also ironic that we talk about this change being made to save screen space, yet a year or two ago you guys did a big revamp of the OneNote UI which included expanding the section list in the Notebooks pane so each section takes up twice as much vertical space as it used to, meaning we have to scroll way further now (and are more likely to have to scroll in the first place) to find the section we want in the list when we have a long list of notebooks. So screen space doesn't really seem to be that much of a concern, yet it is the stated justification for making this unsolicited change. If you want to save screen space, give users an option to revert back to the old compact UI, that would save a whole lot more than this change did. But I digress.
The bottom line is, people here have given more than enough reasons for why this change has had a significant negative impact, that alone should be enough to at least provide an option to move the search bar back to where it was. Rolling back is not an acceptable solution, I think the MS folks already know this - after all, MS has been the driver of the much quicker update cycles we've seen in recent years, and they always want people on the latest version of everything. Well, changes like this are exactly the reason so many people don't want to always update to the latest version of everything - because there has been a consistent pattern of deploying unwanted and unnecessary changes that break workflows.
Hopefully someone on the MS side will receive this message, and acknowledge it along with all the other feedback that folks have given here, and provide an alternative (i.e. open an internal request w/ the ON Team to add an option to move the search bar back).