Q3 2019 still seems way too optimistic to me.
What API design is there, beyond "Linux apps, probably GNOME with libhandy"?
PureOS is based on Debian testing so it is a RR. Aside of hardware there is a problem with baseband firmware because there is no way to change it after phones are built without attaching to the chip itself.
> What API design is there, beyond "Linux apps, probably GNOME with libhandy"?
None. Every internal part is supported in mainline kernel, the only thing not there is the library for calls, UI has 3 options as of yet.
"Library for calls" is there - three options actually. ModemManager, oFono and freesmartphone.org. I've personally contributed PHS8 support to the latter a few years ago when preparing for a different PLS8-based project ;)