XMPP, as Matrix is pretty much centralized, unless you're fortunate enough to register outside of matrix.org. Both xmpp.org and jabber.org is no longer open for registration.
I think Matrix has a lot of smart people involved, and I can't believe I'm saying this, but dear god, are you hiring for a TPM, by chance?
Every, single, time I have to go "Home" -> "Expand" -> "Favorites" to do what used to take one click, I lose a bit more faith in Element/Matrix.
Though, y'all did finally fix the notification sound that made me want to kill myself every single time it violated my ear drums. Only took.... 4 years? for that?