I don't see how anything that changes as rapidly as Linux does could
possibly have it. While it is true the on-disk formats don't change so
often, they *do* change: even ext2 has gained e.g. extended attributes.
(IIRC, at least one of the BSDs moved away from softupdates specifically
so they could add features to the FS without enormous pain.)
Soft updates look like something that would be just the thing to use iff
our programs were written by AIs so we didn't need to worry about
implementation difficulty or degree of coupling. I've long thought that it
looked like it had fallen through a wormhole from the future...