I actually do use this, but it is missing a lot of useful features, some of which seem unimplementable in IRC

One example: the mattermost clients won't register a message as "read" until you switch to that view, so if e.g. I check messages on my phone, I will see unread messages, but IRC doesn't have a way of communicating this between client and server so matterircd seems to mark all messages as "read" with mattermost)