It's a bunch of XMPP software (Conversations, Siskin, Prosody), with minor patches, under a common branding (the Snikket parrot), plus a web portal.
It's aimed at the friends&family use case, with an easy invitation-based onboarding workflow. You can either self-host a Snikket Server (= Prosody + TURN + certificate automation) or sign up for the hosted beta (which is either bring-your-own-domain or a domain under snikket.chat).
Note that snikket.org is not, by itself, an XMPP service where you can just sign up. You either need to run your own instance, or you need to sign up for hosting.
I switched over from irssi to poezio a few years ago (with a self-hosted biboumi as XMPP<->IRC gateway) and I don't miss irssi (anymore).
Of course, the transition was a bit bumpy, but I think that'd go both ways. As someone else said, it's not a direct replica. Yet, it feels close enough and not too close for the uncanny valley.
Possibly not for "political" reasons, but it gets increasingly harder to be an independent mail operator. I've been running my own email cluster for over a decade, privately.
Some mail providers require you to contact them to get on an allowlist to even be able to deliver email at all. Others do not formally require that, but you may have to enroll in a postmaster programme in order to track your deliverability. Yet others there's no hope at all, except running after the most recent fad in spam fighting (like DKIM/DMARC/SPF etc.), only to find that even fully and correctly configured DKIM (which I don't like on my private email for many reasons) is by far no guarantee for not getting your email marked as spam (and thus, on some providers, hidden by default), even from IPs and domains which have had no spam emitted from them for many years.
So... Yeah. It's not heard of in the public media because those affected most by this don't have any leverage. The big mail service providers (MSPs) presumably have their agreements among one another to protect their business, but if you're just doing friends&family, good luck getting anything except an autoresponder when you run into troubles.
(To offset some of the criticism here, I have had very positive experiences with contacting postmasters of some large German email providers when issues happened, so there's that.)
The current edition of those can be found in XEP-0459: https://xmpp.org/extensions/xep-0459.html