I've been using it for almost two years now, and I'm not going back.
It's based on a spare Blackberry Q10 keyboard and a custom Arduino-compatible board that reads the keyboard matrix and outputs it as USB HID to the phone. From the viewpoint of the phone, it's just a regular USB keyboard, so no special software is needed.
But I do use a custom virtual keyboard to have just two rows of symbols that are not natively on the keyboard, as I didn't want to add another layer of rarely used symbols that I'd have to memorize.
(On the image you can see Ubuntu with XFCE4 running on it. I chose Ubuntu because it's what was easiest to get running in a chroot jail on the phone. I'm using VNC to display the GUI. I even managed to get FEX (x86/x64 emulator) and Wine running, so it runs x86/x64 Linux and Windows apps.)
Btw: Is there maybe someone who wants to make a little side money? There are tons of people who say they'd buy this, but I don't want to make them.
The designs are all online and I'm happy to help. So if someone wants to make and sell them, that would be really cool! (I don't want or need any financial compensation. I'd just be happy if people have access to this.)
Yeah, you are right. On the other hand, all the software on there is also FOSS and if you are very liberal with the term "software", design files could maybe also be construed as software.
Tbh, though, I've never seen anyone use the term FOSH.