Some accounts have gone missing, how to merge in lost data from old backup?

I have just now noticed that I have several old (closed) accounts that have disappeared from my Quicken data file. I have a backup from 2020 with the missing accounts in it. What do I do?

*** How do I fix it?
What I want to do is to take the missing accounts from the old backup and merge just those accounts into my current datafile. How do I do that?

*** Likely cause of the issue
I suspect that this is the same issue as this one, where a late 2020 bug in Cloud Sync would cause accounts to be deleted from the Quicken data file (I'm using the Windows subscription version of Quicken):

https://community.quicken.com/discussion/7882617/losing-an-account-from-my-data-file-after-cloud-synch

I think this is the same issue because the timing and problem matches: I see the old accounts in a backup from November 1, 2020 and they are gone in my backup from November 9, 2020. The issue was fixed in December, 2020, according to that post, but the fix just makes this not happen any more, it doesn't restore the lost data, it seems.

If I repair my current Quicken data file (after backup), it comes up with a lot of errors to do with transfers that relate to the missing accounts, saying that these accounts no longer exist.