Downgrade from Safari 12 to Safari 11
Go to Mac App Store page for macOS High Sierra and download the installer.

Create a bootable installer for macOS High Sierra by following the instructions in the support document, How to create a bootable installer for macOS:

Use a USB flash drive with at-leat 12 GB of free storage space and run the following command:

sudo /Applications/Install\ macOS\ High\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume --applicationpath /Applications/Install\ macOS\ High\ Sierra.app


Once you have the installer ready, use it to install a fresh copy of macOS High Sierra. Once installed restore your data from Time Machine backup.

Now turn-off automatic updates in System Preferences and manually check for updates. Under Updates in Mac App Store do not choose to update Safari.