I understand that macOS has its own policies about how various standard Unix directories are used, and that these are (or at least have in the past been) documented in the "File System Programming Guide". I'm about to update to High Sierra (from Sierra) and wonder:

1. have these policies changed in any way between 10.12 and 10.13; and
2. does the update process force changes (e.g. by enforcing new policies or reimposing old ones that have been customized) in organization or existing permissions.

For example I see that it is no longer possible under 10.13 to chown /usr/local/, and I wonder what other changes the update will make to these standard Unix directories in (a) structure, (b) permissions, or (c) my ability to change permissions or organize the directories.