To get old tab bars on Safari for macOS 12, create
/Library/Preferences/FeatureFlags/Domain/Safari.plist
copy the above into it, and reboot.
The feature is controlled by
+[FeatureAvailability isUnifiedBarEnabled]
which calls os_feature_enabled
, which searches in /System/Library/FeatureFlags/Domain/Safari.plist
and /Library/Preferences/FeatureFlags/Domain/Safari.plist
.
I copied the System file, removed the other flags, and changed UnifiedBar to false.
Looks like your file is in your Users preferences, not the /Library/Preferences