All of my Notes disappeared after upgrading to Sonoma 14.3, how do I get them back

Thank you for posting this. I just backed up and thought I completely lost my notes, this is how I found them- I had only one note that showed up under "Google" in my NOTES and the "On My Mac" was so small and grayed out I didn't even think of clicking on it, but my notes appeared again after I did!! Thanks so much!!

My lesson learnt from this issue is -

1. Before you upgrade any OS , keep an eye on Notes. Verify that "On my Mac" is not selected by default option for new notes files.
2. For me, the selected option was "store in iCloud" however my icloud storage was filled and system started creating the notes on the mac rather than on iCloud.
3. When I upgraded the OS, all the notes gone suddenly which were holding very important information.

I tried all the ways to recover it - using backup etc but no win for me.

I have to live with this fact. The important piece of information is lost with OS upgrade.

I empathize with you on your loss. Years ago, there was an OS update that included a change to Time Machine's data structure. Time Machine did an automatic update of the entire disk with the new format. And just as the warning says, when the disk gets full, it dropped the oldest files, two years of them.

One would think developers would code a warning when Time Machine was running out of disk space. Time Machine estimates backup time remaining. That estimate could be based on knowing of the size of the backup. From there, it would be easy to use diskutil to fetch remaining disk space, then perform subtraction. And warn the user if space is likely to be a problem. How hard is that?

I've sent a better worded version of that suggestion to the development community via Feedback Assistant.app. You can find feedback assistant using spotlight if you have an idea you're sure is worth mentioning.