This move of Voice and SMS to an IP protocol is IMO pretty interesting for opensource community because it means we can largely reduce the footprint of the modem to "just" giving IP connections.
The main contributors to opensource IMS client should to be "secure smartphones", long before people doing "let me opensource my baseband", because running a super-privileged SIP client is very dangerous (even Pixels, no matter the OS, got security flaws in it few months ago), but that's clearly not what I'm seeing. (There are also other security considerations justifying an opensource IMS client, like forcing the encryption of VoLTE)
As someone who spend a lot of time making an Android opensource IMS client ( https://github.com/phhusson/ims ), this wiki page isn't really news: I've had this setup (Doubango IMS client, which is a C embedded-leaning opensource client + VoWifi tunnel) running on my desktop using an Android smartphone as a SIM card reader more than a year ago ( https://github.com/phhusson/doubango/blob/master/README-phh ), which is well, just the same age as that wiki page. Except that I've been using it over real carriers, not just software ones.
If someone has some time to spend on it, for GNU/Linux smartphones, Doubango IMS client should be pretty straightforward to get to work, what's missing is "only" plumbing.
Either way, if the people who are working on that are here, please say hi so we can share our war stories :-)