In all fairness, I got my entire non-technical family onto xmpp/jabber with Snikket (which is a highly opinionated packaging of existing xmpp server and clients with minor tweaks). They scan a QR code to download the app and join the server. It is e2ee out of the box and the experience is quite comfortable and seamless, even for a non-technical person.
Can they use it to chat with classmates from high school? Given my parents generation (Boomer, born 1948), they're mostly all on Facebook, so they either use Facebook groups+Messenger, SMS, or email (yikes!). Family chat is just on a giant SMS chain, which admittedly is awkward as there are several "sub-chains" with some people removed.
Or is my family just weird in their chat preferences?
My folks are in the same age group and don't care for FB. Not sure that is representative of boomers as a whole, but the family generally likes the idea of communicating on a private, secure chat system.
I know it's an anecdote, and I'm the founder of the Snikket project, but I rolled out an early version to 10+ family members during lockdown. The majority of them were remote and ages from <5 to >80. I emailed them the invitation link, and that was about it. They all managed to sign up with no issues.
Scanning a QR isn't actually necessary. That flow exists for when you open the invitation on e.g. a desktop or laptop, and you actually want to set up Snikket on your phone/tablet. The invitation page guides you through you what to do in that case, you don't need to know what a "QR code" is. But most people just open the invitation directly on their phone, and it has a magic link to install the app.
In some ways it's easier, because you don't need to go through the SMS verification and stuff that the phone-number-based platforms require. I know various APIs and hacks have made that flow easier over the years, but it's still an extra step for non-technical people to get lost in. It's also an obstacle for people who want to use a tablet, such as my children who are too young for phones.
In any case, I'd encourage people to be less defeatist about the adoption of more open alternatives (intended or not, that's how I read your comment). It is possible... my own family average no more or less technical than any other. They manage to use WhatsApp without help, and they manage to use Snikket without help. And of course I'm not the only person using it successfully.
My mother has no idea how to check her email and doesn't use it. She also doesn't know how to text. Or check voicemail. It's just another point of anecdata, but this is my bar for "non-technical."
I don't know how it would have gone remotely, but no issue in-person. I opened an invite QR on my device, asked parent to open camera, and point it at code. They get the gist that the app name is Snikket, they get notifications of messages and calls, and the layout and icons are pretty similar to every other messaging app. I am going to attempt it with an in-law who only just stepped into the smartphone era this month, has no familiarity with a smartphone interface, and is the least technical person I know.
My dad finally retired his 2008 flip phone last year, but before that a QR code would have been no help to him. I visited them recently, and not a single restaurant (business at all) had QR codes in their town.
There is no technical barriers that can not be overcome to allow the use of XMPP by normal non-technical users. GChat for years was XMPP until google got a market share they desired and choose to take their users off an open protocol in favor of their own one (which they then killed a few years later )
Technical barriers are not usually how non-technical people choose technology - it's usually based on things like UX and how many of their friends already using the service.
I got my entire immediate family on Matrix without too much difficulty. The only big issue happened when my mom got a new iPhone and it cloned the old Element install from her old iPhone, causing two devices to have the same set of identity keys and we had to reset them.
OTR doesn't do groups. So it isn't really applicable to IRC. At this point, I am not really sure that end to end encryption is generally applicable to groups. The identity management problem quickly spirals out of control. I note that Telegram doesn't even try. I think the best that can be done is a scheme that makes everyone entirely trust the moderator of the group.
It's a Signal fork that uses Oxen, a Tor fork. It also changes a lot under the hood with regards to encryption, and I'm not smart enough to know if the changes are good or bad.
There's not a truly solid secure anonymous implementation that I've found.
Matrix (with the Element client), Wire and Threema are a few options that don't require phone numbers, offer end-to-end encryption and have mobile clients. IIRC, the chats also sync across devices (with some duration limits measured in weeks) on all these platforms.