Apple recently announced they are about to cease 32-bit support on macOS coming this fall. Consequently, future updates of Synfire and HN2 will include 64-bit components only.
Until now, Synfire used a mixed architecture approach, where the user interface was run 32-bit while for audio processes you had the option to run either 64-bit or 32-bit engines in order to support legacy and modern audio plug-ins as well.
Note that you will very likely no longer have the option to run 32-bit audio plug-ins on macOS 10.14 and later. Although we are thinking about keeping the 32-bit engine around for a while, obviously, this only makes sense if it will actually work on 10.14. (still to be tested)
We recommend all Mac users begin migrating their audio racks and projects to 64-bit at earliest convenience. There's no reason to hurry, but obviously this will be easier to do while the 32-bit racks can still be opened.
That being said, you can safely dismiss Apple's warning message ("Synfire" is not optimized for your Mac) and continue using the current versions of Synfire on High Sierra. The same goes for Harmony Navigator.
Regarding Windows 10, we will be making the transition a bit later, presumably next year. If you find the time, migrating your racks and projects to 64-bit at next opportunity might still be a good idea.
You'll find detailed instructions on migrating to 64-bit racks here:
(http://users.cognitone.com/wiki/migrating-64-bit)
If you have questions or suggestions, please post to this thread in the forum - thanks.