What happens if you restore your keychain from a TM backup? Does anyone know what happens or what I should expect if I restore all contents of ~/Library/Keychains from a Time Machine backup? I'm trying to assess the risks of doing this.

The reason I am asking is my iCloud keychain, which contains about 8 years of pretty important "Secure Notes", disappeared a few days ago. I believe the reason for it disappearing is that its local copy became corrupt (there is now a files at ~/Library/Keychains/[redacted_guid]/keychain-2.db-
keychain-2.db-corrupt and it was created around the time I noticed problems). I have tried restoring just the ~/Library/Keychains/[redacted_guid]/ folder, but that didn't fix the problem.

A reasonable next step is to restore the entire folder, which I hope has a working db and valid links to it. But I am afraid this will make things worse. Currently, my system is working fine. Using my user password, I can unlock my computer from a sleep and I can unlock the secure parts of System Preferences. I can see things going bad fast if I screw up my primary login keychain.
Answering my own question -- because I simply ran out of time: my need for my secure notes outweighed the risks.

I restored an earlier copy of my entire ~/Library/Keychains folder from a pre-corruption Time Machine backup. This did not restore the "iCloud" option in my Keychain Access "Keychains" panel, but the secure notes all reappeared under "Local Items". After restoring the items, I was able to lock and unlock things in System Preferences, and I was able to log-out and log-in to my computer (after making copies of all the secure notes, and so far I can't say I see any obvious problems.

So in my case, the answer to my question is: it didn't do anything bad, though it will (of course) reset any passwords you may have changed since the backup.