Facebook makes major interface changes fairly frequently. I'm not adverse to change, but I'd like to know what's changing and why. Is there an public, up-to-date Facebook changelog?
You can find them in their blog - https://developers.facebook.com/blog . As usual they post list of issues\changes that were released
And also there is a roadmap for future changes, that can be found here - https://developers.facebook.com/roadmap/
No. There is no official public facing documentation of user interface changes to Facebook, even when those are quite significant (ex: May 2016 changes to behavior of Facebook Groups and Group Suggestions).
I have not been able to find a good 3rd party resource for change publication either.