@kino @kroner @MMS21 @Burn Imagine how many people are on Matrix.org, and how many who are on other instances interact with at least someone on who's on Matrix.org.
Unlike XMPP that keeps data local, Matrix actually makes full copies of all the data to each instance it gets in contact with, so in theory, even if you have no contact with anyone on Matrix.org at all and your entire MUC consists of nobody who's on Matrix.org, but somebody who's in your MUC has a friend who's on Matrix.org, then that metadata still gets sent to Matrix.org.

I haven't used Matrix for years now, so this is basically theory.