The register keyword was depreciated in c++11, and is removed in c++17. When c++17 is enabled In Xcode 8.3 (via -std=c++1z), JUCE becomes uncompilable due to all of the register uses. I have not tried in other platforms, but this needs to be addressed moving forward.
We’ve never used it in our own code, but I guess you must be talking about its use in some of the 3rd party code we include? Thanks, we’ll have a look.