I don't really get what is wrong with how it was:
"As I mentioned already, use systemd drop-in file (see `man 5 systemd.unit` for more details). Or use `systemctl edit --full slapd.service`."
As previously mentioned, you will need to ask RedHat their reasoning.
The quote is part of the answer of redhat. I am not sure if it makes sense to ask any further there. Especially since I am not really familiar with this transition wanting to run everything via this systemd.