You dont need to install mysql server to use libcod. It includes a connector which can connect to mysql servers, but it's not a requirement. If you read the installation guide, you'll see that it requires mysqlclient, not mysqlserver. You can also disable mysql inside the config files to get rid of it entirely.
It's not ddos protection for YOUR server, it's protection from the q3 getstatus/getinfo amplification attack, in which your server is utilized to amplify a ddos against a third party server.
For scripting, libcod adds multiple script functions not available in stock cod2_lnxded.
And multilib is not uninstalled by libcod, it's uninstalled by the prereqs for compiling your own libcod. Voron's version already uses g++ only, you dont need gcc anymore.