After demoting a domain controller, the related event logs (DNS Server, File Replication Service, and Directory Service) are not removed. While this is technically not an issue, it can potentially confuse monitoring systems (SCOM inadvertently detects a DNS server while there is none). Or, perhaps you should want to be tidy.
Although the change is effective immediately, the underlying event log files are still in use. If you wish to remove these, you'll need to reboot the server in order to have the Event Log restarted (you cannot manually restart the service).