An additional option which is now available is to host your own Mattermost server. GitLab has integration for it even.
IRC is still my preference, but it takes a fairly technical person to appreciate it and host their own server. If you're catering to more general people, having 1 technical person spin up Mattermost in docker might be a good solution.
Does Mattermost solve the last problem with Slack listed in the article? This seems to be the single most important complaint. Anything that can be described "hey, it's just like Slack, except it's FOSS" is still going to have that problem.
Namely, is it still intended primarily for intra-team collaboration, or can it handle communication with the public as well?
It actually is included with the gitlab omnibus installation package now, and then it uses gitlab's LDAP for logins. I just had to set a few options in gitlab.rb because I wanted to use SSL and to set the external hostname.
IRC is still my preference, but it takes a fairly technical person to appreciate it and host their own server. If you're catering to more general people, having 1 technical person spin up Mattermost in docker might be a good solution.