Debian/buster is the current stable release, there's no point in
enabling stretch-backports there.
FTR: we could set it to 'stable-backports' instead, though then
apt/apt-get/... will complain about it:
| W: Conflicting distribution: http://ftp.debian.org/debian stable-backports InRelease (expected stable-backports but got buster-backports)
deb-src http://security.debian.org/ stable/updates main contrib
# backports:
- deb http://ftp.debian.org/debian/ stretch-backports main contrib non-free
- deb-src http://ftp.debian.org/debian/ stretch-backports main contrib non-free
+ deb http://ftp.debian.org/debian/ buster-backports main contrib non-free
+ deb-src http://ftp.debian.org/debian/ buster-backports main contrib non-free