Right now there's a Note History setting that controls how many days until a note expires. The problem is that this is stored clientside, so if you set it on one client, but not on another client, your second client will continue dutifully expiring notes without any warning. This is completely hidden behavior (unless you happen to check the documentation page, which I think few people will) and also introduces questions about how you log in another client; what am I supposed to do, remember to rush to the setting before it starts deleting old notes?
This is clearly an account-wide setting, not a client-specific setting, and should be treated that way.
I believe it controls how many days of note history depth is retained … on that client. Not elsewhere. I.e., you can have a note history on one client that is rather lengthy, and a note history on another that is short or non-existent. And that client with the longer history will retain its context of that note that far back in time. And each client will have a varied context depending on syncing and lots of things.
IMPORTANT: Please note that since all the revisions are synced across all devices, it means these settings are kind of global. So for example, if on one device you set it to keep revisions for 30 days, and on another to 100 days, the revisions older than 30 days will be deleted, and then this deletion will be synced. So in practice it means revisions are kept for whatever is the minimum number of days as set on any of the devices. In that particular case, the 100 days setting will be essentially ignored, and only the 30 days one will apply.
Assuming that page is correct, then this is an issue; if that page isn't correct then it should be corrected.