A public Matrix server. Its just a never ending black-hole of ever increasing storage requirements and the software is too buggy to not become a maintenance hassle.
I do run a Synapse server for bridging purposes, so I am not just talking in theory.
And so damn easy to self-host in general. Ejabberd is batteries included down to offering stun/turn for audio/video calls, Erlang is just unrivaled when it comes to hot reloading so updates are effectively zero-downtime (unsurprising considering all the business critical environments it's deployed).
At first (and especially because I went with Matrix originally) I wouldn't think of self hosting all my instant messaging, but in retrospect, ejabberd is one of the easiest services I've got to maintain. I highly recommend everyone to give it a shot, especially to all the matrix refugees to whom it was a surprise/disappointment.