The following notice was in a newsletter from Laura Shoe's site:
I want to give a quick but important heads up to Mac users: there are very serious incompatibilities between old versions of Lightroom - that is, Lightroom 5 and earlier - and Apple's new Sierra operating system that will be released this Tuesday, September 20. If you are running an old version of Lightroom, I strongly recommend that you not update to Sierra when it's released.
Adobe has been testing Sierra with Lightroom 6 and CC, but often there can be issues that don't surface until a large number of users are running the new operating system with Lightroom and are in effect testing them. It's therefore not a bad idea to hold off updating your operating system until any other major issues are surfaced and fixed, or until it becomes clear that any issues identified won't affect you personally. Once Adobe posts its link to known issues, I'll share this with you so you can stay up-to-date.
