Hi,
On the Wireguard site the Debian installation Pin-Priority is at 150 - this
is too high because the official Backports (Stretch-Backports currently)
has a default of 100. This makes certain packages such as kernel images to
be pointed to Unstable instead of Backports.
I believe a number such as 90 should be better choice.

For example for the package linux-image-amd64 we have:

# apt-cache policy linux-image-amd64
linux-image-amd64:
  Installed: 4.18+98~bpo9+1
  Candidate: 4.18+99
  Version table:
     4.18+99 150
        150 http://deb.debian.org/debian unstable/main amd64 Packages
 *** 4.18+98~bpo9+1 100
        100 http://ftp.debian.org/debian stretch-backports/main amd64
Packages
        100 /var/lib/dpkg/status
     4.9+80+deb9u6 500
        500 http://ftp.debian.org/debian stretch/main amd64 Packages
        500 http://security.debian.org/debian-security stretch/updates/main
amd64 Packages

But if we lower the Pin Priority to 90:

# apt-cache policy linux-image-amd64
linux-image-amd64:
  Installed: 4.18+98~bpo9+1
  Candidate: 4.18+98~bpo9+1
  Version table:
     4.18+99 90
         90 http://deb.debian.org/debian unstable/main amd64 Packages
 *** 4.18+98~bpo9+1 100
        100 http://ftp.debian.org/debian stretch-backports/main amd64
Packages
        100 /var/lib/dpkg/status
     4.9+80+deb9u6 500
        500 http://ftp.debian.org/debian stretch/main amd64 Packages
        500 http://security.debian.org/debian-security stretch/updates/main
amd64 Packages