After Flutter was updated by HomeBrew on my Mac, I could not test a Flutter mobile application on a real IOS device connected to the MacBook via the cable. I was getting a pop up from the system saying that iproxy could not be opened because the developer could not be verified. To solve this error run the following command:

$ sudo xattr -d com.apple.quarantine /path/to/flutter/bin/cache/artifacts/usbmuxd/iproxy

But I don't know my path to flutter !?

No problem!

First we'll find where the flutter command is actually located:

$ which flutter
> /usr/local/bin/flutter

And now that we know where the flutter binary is actually located we can perform a long listing of the binary which will show us that its actully a symbolic link to the Flutter installed by HomeBrew:

$ ls -l /usr/local/bin/flutter
> lrwxr-xr-x  1 username  admin  53 Dec 14 11:33 /usr/local/bin/flutter -> /usr/local/Caskroom/flutter/2.8.0/flutter/bin/flutter

This command is showing us that Flutter is actually located in : /usr/local/Caskroom/flutter/2.8.0/flutter/bin/flutter

Therefore the actual command neede to fix the iproxy issue is:

$ sudo xattr -d com.apple.quarantine /usr/local/Caskroom/flutter/2.8.0/flutter/bin/cache/artifacts/usbmuxd/iproxy

Conclusion

Basically all you need to do is to find where your flutter binary actually points to and use that path to remove the iproxy from the Apple quarantine.