I use it daily :) The Conversations app works quiet well on Android. I also use Profanity on my desktop, but most probably wouldn't like it because it is a command line application. I've heard good things about Gajim for those interested in a graphical desktop application.
Dunno if keybase can really be considered to be based on PGP ... Many XMPP clients support PGP but these days OMEMO is the hotness for standards based messaging. A list here:
My current fave XMPP client supports PGP messaging and runs on lots of desktop platforms:
I will leave it as an exercise to find good android/ios clients that support PGP:
You just need to set up a server and host an XMPP multi user chat (MUC). It's moderately difficult, but not rocket science.
Anyone can do it, and the servers federate, so as a network it is highly resistant to government censorship.
Just know that it isn't an "anonymous hide me" technology.
Some people will say Matrix chat is the solution. I disagree, but only on a basis of suspicions. The Matrix protocol and primary instances are highly centralized.
We're happy to answer any questions you might have about it! And in the meantime, here are a couple more resources based on the OP's queries:
If you prefer a website client you can login to your XMPP account at https://nl.movim.eu/ . And for a desktop XMPP client we recommend https://gajim.org/ . There are very good mobile clients too (like Conversations and Siskin).
All of the above support the full set of JMP texting features, including text messages (to/from any country in the world), picture messages, group texts, and short codes, as well as receiving calls (see Q17 at https://jmp.chat/#bot for the steps to enable that option, aka Jingle). You can place calls using a SIP client like CSipSimple or Linphone.
Note that (per above) JMP supports group text messages, which is a very difficult feature to find with other services. So if group texting is important to you, please do check the alternatives to confirm they have group texting if you end up going with a service other than JMP.
If you just want to chat via XMPP, I would recommend to forget about Cisco Jabber.
There are various clients available, a selection can be found here:
https://xmpp.org/software/clients.html
I don’t use Windows, so I don’t know if there’s a better one for this OS, but I can recommend Gajim.
Yeah, this sucks -- I guess if you are Jack Dorsey you could afford a burner phone with a secret number for this sort of thing.
There's also this (https://gajim.org/) which I'm toying with as a means of doing sms from my desktop at the moment, but it's kinda beta at this point and I'm not sure I'd trust it for anything important either.
I'm not sure what you're trying to prove with your example of elderly people. They don't seem to be able to use the internet at all, so I'm not sure a whole lot of their privacy is at stake here. This is besides the point though, the extremes of ages are going to need assistance, this is unavoidable.
If people can't remember another username and password then their time online including use of social media, shopping, bank accounts and so on are also at stake. There are also ways of helping them to remember passwords such as using a password manager or phrasing passwords so they are memorable.
iOS is going to receive OMEMO support through ChatSecure in the coming weeks, Gajim also exists on Windows and OSX, though the later admittedly isn't the easiest task to deploy.