Why is there no such command in elementary to clear the DNS cache?
sudo /etc/init.d/dns-clean start command not found
Since elementaryOS 5 and newer,
systemd-resolve handles flushing the DNS cache.
This was an upstream change made in Ubuntu 17.04 according to this AskUbuntu answer.
Use the following command to flush your DNS cache.
sudo systemd-resolve --flush-caches