I’m assuming your 6.0.4 project had the “macOS Base SDK Version” field set to “10.12 SDK” for each of the configurations of the Xcode exporter.
I tried saving a project using the 6.0.4 Projucer with the base SDK set as above, and then resaving it with the 6.1.2 Projucer. I was then able to open the project in Xcode and build it without problems. (I did have to manually install the 10.12 SDK first, though). I’m using Xcode 12.4 on macOS 10.15.7. It doesn’t seem like anything has changed in JUCE that would introduce the error you’re seeing - however, without knowing all of your project settings it’s hard to say for certain.
Disclaimer: I don’t recommend doing the following! Using the newest SDK is almost always the best approach.
To build against an old SDK (e.g. 10.12) in a recent Xcode, you need to add that SDK to
Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs. You also need to update one of the plists in the Xcode bundle to set the minimum SDK version:
sudo /usr/libexec/PlistBuddy -c "Set :MinimumSDKVersion 10.9" /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Info.plist
Whenever Xcode is updated, you’ll also need to run these steps again.