It is not just IRC ports. Back when I wanted make my own MMO and went shopping for business ISP, I noticed one of the contracts I read even wrote a long list of all possibly IRC related stuff, they would ban you if you had: irc server seevice, irc server software irc client software, irc bots, tools to make irc bots, mirc scripts, and a bunch of other stuff I don't remember (this was around 2007)
Exactly. All of these would be part of running a botnet. In the 90s and early 2000s you assuredly had to seek out hosts specifically designed for hosting IRC shells or services. Nowadays that DDOS and traffic management work better I see less of it.
