networkmanager-sstp was released June 29th: https://sourceforge.net/projects/sstp-client/files/network-manager-sstp/ Latest in portage is 0.9.8 which is incompatible with networkmanager 1.2.x
commit ddf1ea87c233a632e6e6c4137b98da733d1ebaab Author: Sergey Popov <pinkbyte@gentoo.org> Date: Thu Sep 15 11:52:56 2016 +0300 net-misc/networkmanager-sstp: version bump, drop old Reported-by: Sven <sven.koehler@gmail.com> Gentoo-Bug: 589596 Package-Manager: portage-2.2.28