Realistically it's hard to beat IRC for low barrier to entry. Most major networks have web clients now, you pick a handle, type a hashtag, and you are in. The signup process for Discord makes me roll my eyes every time I have to use it. Also, the textual nature of the IRC interface and lack of Fisher-Price widgets and visuals to entertain the short attention span millennial makes for a more pleasant conversational experience.
After IRC we switched to SILC for many years. It was quite nice and very IRCish (+ crypto), but after a long time without new development and maintenance, we went with matrix. Two years ago it was still a bit bumpy. A lot of work went into it and now it's mostly smooth sailing -- can recommend.
community leader here, we have 500 or so people on the network, I can give my reasons.
IRC vs Slack:
No brainer, Slack is a hosted application that can change its client on a whim. Bot functions are nice but moderation tools are severely limited and the client is very heavy.
IRC vs Discord:
Similar to Slack in nearly all regards, however it's better at being what slack was supposed to be I think. It definitely is "eating my lunch" compared to Slack. But the same rules apply, it's very heavy and you have no control of the client.
IRC vs XMPP:
XMPP is great. The clients for it are relatively good, the protocol, while XML-y is good, but the bot tools are not very good, and federating is hard given the XEP fiasco.
Additionally, most existing clients treat XMPP as a IM platform and do not focus on chatrooms.
Why IRC then:
Well, the truth of it is that if you're sitting on a wired, stable connection then IRC as a text based instant messaging chatroom system is really good. It has no "frills" like emoji responses or threading. No avatars, nor does it force your client to render anything in a specific way. You're free to use what you want.
If you want inline images, there's a client that does that.
If you want to have a persistent presence, searchable backlog then there's bouncers, or a managed service that does that[0].
If you want a terminal experience that is lean and stripped, or a big fat client that grabs gravatar emails from nickserv registered accounts to display them as avatars, it's possible, anything is possible. And that possibility also exudes the hacker culture.
IRC also does not require an account of any kind, this can lead to some measures of abuse, but due to the very good (though, admittedly hard to use) moderation tools you can have many different ways of managing the community, from shadow banning people, to muting channels, to invitation only channels or passwords- all the way to channel or network level blocks on usernames/IPs.
Also: nostalgia and I've configured the client how I like it[1]
I wonder what are the advantages of that over newer technologies like Slack, Discord, or even XMPP.