As of now SSL certificates used by SSTP VPN are only allowed to be stored in /config/user-data/sstp this is in contrairy to othe SSL based VPN solutions (OpenVPN) where the key can be stored anywhere on the system. If the user stores the key outside of /config a warning is shown indicating that the key will be lost on upgrade.
Switching from a relative path in /config/user-data/sstp to an absolute path also gives the advantage of using certbot for SSTP VPN as the cert can be directly referenced.