A lot of people use Signal. It may not be the best solution out there, but it is so, so, so much better than the proprietary alternates.
One good thing is that a normie can easily use it as an alternative to WhatsApp, since the app design is so similar. I mean, it is easy for family and friends to understand and start using Signal, compared to something like Matrix or XMPP.
And if someone needs a little more hardening, they could use the fork called Molly, which has a few more security benefits over the stock app.