Log Message:
The shell backend was retired, the sock overlay should replace it

If you had this:
database        shell
suffix          "cn=log"
rootdn          "cn=root,dc=log"
add             "/usr/local/bin/prog"
modify          "/usr/local/bin/prog"
delete          "/usr/local/bin/prog"
modrdn          "/usr/local/bin/prog"

Now use this (not documented anywhere!)
database        null
suffix          "cn=log"
overlay         sock
socketpath      "/var/run/prog.sock"
sockops         add delete modify modrdn

And now you need to write a daemon that reads on a local SOCK_STREAM
socket to get the data (the format is the same slapd-shell produced),
and executes /usr/local/bin/prog with that data on stdin.