The traditional tool for digital communication at CCC congresses is IRC. I personally still like IRC, but I think the matrix web client at FOSDEM made channel discovery and navigation a lot easier.
I also think promoting Matrix could help with promoting open ecosystems in general. IRC works well, but it doesn't have the potential for mass adoption, which we need in order to curb WhatsApp's influence.
We tried to give visitors the freedom to use their preferred tools to join FOSDEM.
Whether you were using matrix with the latest Chromium or irssi in a tmux session on OpenBSD, you were able to speak to people at the conference.
Same for the streaming. Inside the matrix channel would have worked, but mpv on some framebuffer without X or wayland too...