Are you certain? The ln command, when run without -s, would return an error if you used it under Tiger or earlier?
Or are you referring to hardlinking to directories, which was something UNIX traditionally supported, but which required root permissions (as it was used by the mkdir command to create the . and .. directories), and which was removed at one point (4.2BSD, as that added the mkdir() system call, making the ability of link() to link to a directory unnecessary?), and added back in 10.5 with the introduction of Time Machine, so that it could be used in backup trees as a very hacky form of de-duplication (each backup tree is a complete copy of the file system being backed up, but if there's an older copy of an unchanged file or a directory everything under which is unchanged, the "copy" is done by making a hard link rather than by copying the file to the backup disk).
Instead of tackling "bitrot" head-on, Apple seems to have taken the "make backups easy" approach. This works to some degree, but since the backups use hardlinking, you really only have two copies of the data -- the one on your main drive, and the one on your backup drive. This makes cycling your backup drives even more important than it already was.
That's what happens with any backup scheme that does incremental backups - if a file hasn't changed, a copy isn't made.