Is it supported? Short answer: No, it isn't supported.
From https://wiki.debian.org/SystemDowngrade.
It's still possible but:
WARNING: BACKUP YOUR SYSTEM FIRST.
Replacing all testing with stable.
# cat /etc/apt/sources.list
deb http://deb.debian.org/debian stable main contrib non-free
deb-src http://deb.debian.org/debian stable main contrib non-free
deb http://deb.debian.org/debian-security/ stable/updates main contrib non-free
deb-src http://deb.debian.org/debian-security/ stable/updates main contrib non-free
deb http://deb.debian.org/debian stable-updates main contrib non-free
deb-src http://deb.debian.org/debian stable-updates main contrib non-free
Running apt update && apt dist-upgrade now wouldn't change any packages, because packages from testing have newer version numbers than the ones from stable.
To circumvent this, create /etc/apt/preferences.d/downgrade with the following content.
This file can be deleted after the update has completed.
# $ cat /etc/apt/preferences.d/downgrade
Package: *
Pin: release a=stable
Pin-Priority: 1001
Now, Debian will install packages from stable.
WARNING: BACKUP YOUR SYSTEM BEFORE THE NEXT STEP.
sudo apt update
sudo apt dist-upgrade
Cleanup.
rm /etc/apt/preferences.d/downgrade