Find all my backed up photos on Time Machine drives
Aha! Here's the answer: https://superuser.com/a/1591350/2684
On Big Sur (macOS 11), you need to go to System Preferences and enable Full Disk Access for the Terminal (or, iTerm, in this case). Then, magically, a command like:
chmod &quot;-a#&quot; 0 Backups.backupdb/

works just fine.

I also learned that chmod -N will removal all ACLs from a file, via https://superuser.com/a/299912/2684 .