After Security Update 2019-003, OpenLDAP is corrupted. How can I recover?
I have been able to repair this. It may have been luck. What I did:


Copy all private/var/db/openldap/authdata from all Time Machine
backups to a working directory (for i in 2019*; do ditto $i/DumbledoreRoot/private/var/db/openldap/authdata/ /tmp/$i/authdata; done). My boot volume is called DumbledoreRoot.
Run crash recovery on all the authdata copies, to see which one are
recoverable (for i in 2019-0*; do echo $i; db_recover -cv -h $i/authdata/; done)
Most would report an error. This is not strange, Time Machine cannot
really backup this data as backing up a live database mostly gets
incorrect results in the backup. I do wonder how restore from a Time
Machine does this, though. There must be a better way.
Replace /private/var/db/authdata with what is in that backup. I
probably should have copied openldap-data as well, given what
happened next
Ran slapd -T test and noticed there still was some corruption (though different):


Output:

bash-3.2# /usr/libexec/slapd -T test
5cdff90d bdb_db_open: database "dc=Dumbledore,dc=local": unclean shutdown detected; attempting recovery.
5cdff90d bdb_db_open: database "dc=Dumbledore,dc=local": recovery skipped in read-only mode. Run manual recovery if errors are encountered.
5cdff90d bdb_monitor_db_open: monitoring disabled; configure monitor database to enable
5cdff90d bdb_db_open: database "cn=authdata": unclean shutdown detected; attempting recovery.
5cdff90d bdb_db_open: database "cn=authdata": recovery skipped in read-only mode. Run manual recovery if errors are encountered.
config file testing succeeded


Despaired. Then on a whim, started Open Directory from Server.app and to my surprise, it started and my network user accounts were back.

So, I'm probably just lucky and better answers may help future problems.