ando@sys-net.it writes:

I didn't mention that the whole discussion started because Debian seems to ship with both a default DB_CONFIG installed in the database directory, and the very same directives echoed in slapd.conf.

The current Debian packages do not do this so far as I can determine.

They do install a DB_CONFIG file when *upgrading* the slapd package, but only if a database already exists and no DB_CONFIG file exists, since that's known to cause extremely poor performance. A new installation of the slapd package does not create a separate DB_CONFIG file in the database directory. It puts dbconfig parameters into slapd.conf and then lets slapd write a DB_CONFIG file.

Perhaps you're only looking at the database directory after slapd starts, at which point slapd itself has already written out the DB_CONFIG file?