It's the last release they made any significant updates to the BSD userland. I'd mark this as the release they ceased serious investment into the operating system itself. After this point it's almost nothing of note. If you look at the dates of the various tools etc., this release is when they were last updated. Many are now getting on for being a decade out of date. The only major change has been the switch to llvm, and they made that horribly painful.

It also marks the decline of the desktop UI to introduce increasing amounts of iOS-like behaviour and appearance to the detriment of a usable desktop. Like proper scrollbars etc.

While some nostalgia might account for holdouts, it was the peak of MacOS in the minds of many, including myself. As a developer, I've been quite disappointed by its direction and declining quality. For the amount we pay for this hardware, it's not much to ask for some basic maintenance work and testing to be done.