I had to write my own schema for my application and I was struggling to figure out how to import the schema I wrote to cn=config. I had though I was missing something until I found this.
https://gist.github.com/jaseg/8577024
Notice the name of the script. Someone had to write this because there's no tool that you can use to import schemas when the slapd is configured with cn=config. You can get away with it by using include directive with slapd.conf. I know there's a way to do it with slaptest, but that method is just unacceptable. So, how I understood is, either you have to use slaptest to rebuild the entirety of the schema in ldif format, or you have to handcraft the schema in ldif by figuring out with {} index to start with. This is all backwards.
slapadd and slapmodify are just cranky tools you can only use in specific cases(eg: restoring from mdb_copy backup data). In most cases, using those tools to manipulate cn=config is near impossible.
I like the idea behind cn=config, but I believe that calling slapd.conf deprecated is just unfair with problems like these. Is there a set date/version for the drop of support for slapd.conf? I'd like to stick with slapd.conf if it's not going to happen in like 5 years time.