Best other options are Matrix (mentioned below) and one of the encryption solutions on top of XMPP (OTR, mentioned below, or OMEMO, which is probably a better c

Best other options are Matrix (mentioned below) and one of the encryption solutions on top of XMPP (OTR, mentioned below, or OMEMO, which is probably a better choice in most cases). Both of those are federated, as well, which Signal doesn't offer.