There are some kinks to work out with a few pre-installed apps (glacier-gallery and glacier-filemuncher most notably), but the UI works.
Tested in Qemu and on PinePhone, and it's remarkably smooth on the latter!
This is Glacier, a fully FOSS replacement for the proprietary UI on SailfishOS. However, it's not limited to SailfishOS, and porting this UI to postmarketOS involved a lot of packaging of various Mer components and upstreaming a lot of Musl and non-systemd fixes to Jolla. Based on the Mer stack which is maintained and updated by Jolla, and built with Qt5 and QML.
It most definitely isn't as ready or polished as Phosh and Plasma Mobile are, but it's usable and promising.