Thanks for that. I know that the inode structure doesn't, or at least didn't return this, for a long time (occupational hazard: outliving your education's "best used by" date). Which was why I'd already edited the initial "most" to "many" describing filesystems above (prior to posting).

I'll still maintain it's not universally available on filesystems. And is unreliable (many allow changing this value, see, e.g., touch(1)). And that unallocated space lacks filesystem metadata.