They switched to Qt 5 when they had to switch from using ST-Ericsson to Qualcomm and found out that there is no way they can get X.org drivers from them. So they started using libhybris with Android drivers and Wayland on top, and that by default implied Qt 5.

Just found this page: https://sailfishos.org/about-technology.html

They use Qt 5.

The real question is when are they going to announce a total rewrite of the OS that will obsolete the shipping version. Usually they announce that right about the time the phone ships.

By that reasoning, Android 4.4 obsoleted 4.3.

I'm not going to explain the joke, but Android is backwards compatible.

the joke is that this is what Nokia did almost every time they released a NIT OS (ie maemo) version.