Has Unix directory usage or policy changed in High Sierra?
No super big changes on directories is my feeling / opinion.

High Sierra changes very little with respect to SIP protection and you can easily use ls to read ACL and restricted status protecting from root / super user modification of files.

Everything that was changed materially was versions before. High Sierra does introduce APFS and the concept of snapshots and radically more efficient storage and access speeds - but on the UNIX directory level - this is a snoozer of an upgrade.

Your /usr/local change is the only one that's notable - you can't get rid of it* but you're free to roam within it and change things to suit your needs.

Apple provides additional details about SIP Protected directories.

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* Of course, you can still get rid of SIP protected files / folders / directories by disabling SIP by doing the hokey-pokey of reboot, disable, reboot, change, reboot, re-enable, reboot (Assuming you want SIP in general but not for this one thing). But then, the next time an OS update comes, your work customizing the OS could (and eventually will) fail since the installer will then put the file back where Apple intended it to be - existing and protected by SIP.