Can I safely delete a “.keychain” file from Keychains when there is a corresponding “.keychain-db” file? In the folder “Keychains” in my Library folder (~/Library/Keychains), I have a file “login.keychain” and a file “login.keychain-db”. Is the file “login.keychain” still being used in macOS Sierra, or is it safe to delete it?

I have a few other keychains with pairs of these files “name.keychain” and “name.keychain-db”. But when I create a new keychain in Keychain Access, there’s only a file “name.keychain-db”. Based on this and the modification times of the files, I’m guessing Sierra only uses the new files (“name.keychain-db”), but that the upgrade left the old files (“name.keychain”) in place as a backup? So it should be safe to remove the old files?
Yes, the .keychain files are the old format.

.keychain-db is the new format.

Modify a key in Keychain access and observe the modification dates. I deleted those files years ago without issue.