How can I repair a keychain in Mojave?
The repair tool was removed over security concerns where it could be use to export your keychain without your permissions. what we're left with are guidances you'll find throughout various blog articles explaining some specific problems with workarounds to "fix" specific issues related to the keychain.


How to Resolve Mac OS X Keychain Problems
5 Common Keychain Problems on Mac and How to Fix Them
Keychain Access Mojave


Most of these solutions involve deleting specific offending keys or toggling "Show Summary" or other aspects of a problematic key.