https://matrix.org is essentially a modern IRC, but federated and completely FOSS. I suggest people drawn to an IRC-like experience without the proprietary nature of Slack and Discord [give it a shot](https://riot.im).
Maybe the intended contrast isn't between IRC and Matrix, but between Matrix and Slack (which a lot of people like to call "modern IRC" and which isn't FOSS).
Riot is the only stable client officially recommended on Matrix's website, and it's in-browser.
So it's not really a viable alternative for a lot of IRC users.