I participated and spoke. As a user, it took a moment to wrap my head around the setup, but in retrospect, it was probably one of the best virtual conference setups I’ve experienced in the last year (and I ran a virtual conference using Zoom, Slack, YouTube, and Twitch last year).
I did not participate in FOSDEM actively, but one thing I learned from it that Matrix and Element are really good these days. I remember looking at riot.im two years ago and being underwhelmed; now I am impressed, it feel like a wonderful evolution of opensource messaging, teeming with life, seamlessly bridging Freenode and gitter.im; it’s like I am back to the wonderful days of Jabber.
I assumed Matrix can do video streaming by itself. Seems like you still need another service, e.g. Jitsi.