I'm a long-time user of OFTC, Freenode, and now Libera Chat. As far as I remember, IRCv3 surged around 2012 on the initiative of some Atheme devs (Freenode's official services by that time) who wanted to modernize the protocol. There were some contributions from OFTC folks too. This was way before the massive migration to Libera Chat in 2021, which was triggered by the hostile takeover of Freenode by the Private Internet Access founder¹. There's lots of info about their working group right on a dedicated page² on the OP-linked website.

[1] https://en.wikipedia.org/wiki/Freenode [2] https://ircv3.net/wg