I can finally confirm that adding the SUPL_VER key to gps.conf and rebooting the device afterwards makes SUPL requests work for 4G network connections on the latest Xperia 10 Mark 3 device, too!
Here is a screenshot from go-tcp-proxy intercepting the requests between phone and SUPL host.
In my case the secure SUPL transport was already disabled after I initially flashed SailfishOS so you might not need to patch anything at all! However to make sure that secure SUPL transport is disabled you can simply use the --disable flag. suplpatcher will first check if the setting has been already disabled and if not changes it.





