I, too, recently lost about a decade of notes from my iPhone. I am documenting the case here, for my own benefit, and in case it helps someone else. Recently (~Sept/Oct 2023) all of the notes on my iPhone 11 disappeared. These were notes in multiple folders that were stored "On My iPhone", not in "iCloud". [Note that the distinction between saving notes "On My iPhone" vs "iCloud" is ~relatively~ new. I started saving notes before that was an option which, I guess, is why they were "On my iphone" instead of on icloud; I never changed their location because I had no reason to. I have a family share backup plan so assumed they were backed up but apparently that's not the case -- notes "On My iPhone" are not backed up to icloud; so, if you're reading this, you may want to consider moving yours to icloud].
My notes disappeared just prior to updating to iOS 17, so the loss must have occurred under iOS 16x. I know this because (a) although I have my iphone set to, and consistently get, automatic iOS updates, iOS 17 did not auto-date; and (b) when I first called Apple support about the loss of my notes, I was told that upgrading to iOS 17 might fix the problem; so i did that but unfortunately the notes did not reappear. I just got off my second (2+ hours) call to Apple support which also failed to find the notes. At this point they are saying they are likely lost forever.
I should mention, vis-a-vis the original poster (mr-ginge), that I have no storage-space issues on my iPhone or iCloud account. I also have not linked my Notes app to another email address (an issue discussed in Appleville when Notes go missing). And there is no way I accidentally deleted mutliple folders of Notes I have had and accessed for over a decade.
One other detail potentially worth mentioning is that I have two Apple IDs; my first (call it ID1) was established many (~12?) years ago, and my second (ID2) was also created a long time ago (~8? years; when the me.com domain was available). ID2 is the only ID i really use, although there have been a few times when I needed to log in with my old ID1 account, usually for older apps or purchases -- books or music -- I bought under my previous/original ID1. Along these (2 Apple IDs) lines, when I currently go to Settings > Notes (logged into my iPhone under ID2), there is an "Accounts" setting; mine shows "2" which, clicking on that, reveals them to be my ID1 & ID2. So the iOS "knows" (at some level) that I have saved notes with both IDs. Not sure this matters because, until 2 months ago, I was able to access all of my notes in the Notes app when I was logged in under ID2 (but maybe one can be logged into Notes with a different ID, like my ID1?). When I click on ID1 account, I see an iCloud field which shows "5 GB" which is the original, free storage level that I had with ID1, prior to my current family share plan of 200 GB with ID2. The 2nd Apple rep i spoke to today (Mike) had hoped that my old/lost notes were being backup via that old/free storage plan (under ID1), but it shows only 15k of storage being used so apparently they're not there. But get this...
If I go to Settings > General > iPhone Storage > Notes, it says that, in addition to the "App Size" (11.7 MB), I also
have 40.8 MB of "Documents & Data". And, since losing all my notes, I have not added any new notes "On My iPhone", so what is using that 40.8 MB of storage? This suggests to me that my lost notes are still stored on my iPhone, I just can't get to them. I plan to investigate this further and will update if anything comes of it.
