I use IRC quite heavily. Especially for cryptocurrencies and local hackerspace channels. They're all very active.
But I agree with the post, "irc is not fine", and never was fine if you want to communicate with "regular" people.
It doesn't have spam filters, easy GUI or easy registration process. You could build all of those on top of IRC, and I'm sure that some people did, but there's no "standard" and most people just can't be bothered.
As the post said, often "IRC traffic isn’t allowed past institutional firewalls".
Besides, when was there a time in the history of the internet that irc was used to communicate with laypersons? If that's Mozilla's goal, they're right to switch.
That said, irc is fine, or even the best, for some communities. Especially communities of internet-savvy people.
It's decentralized, open, easy to access and inclusive. Such communities rarely meet the need for spam filters, and each of their members possess a well-distilled taste for personalized tools for irc.
That's why cryptocurrencies or hackerspaces channles are great on irc. That's why #rust channel is fantastic.
I still didn't find any modern substitution as good as irc for those communities. And yes Matrix, I'm looking at you! You're just not there yet.