NVIDIA doesn't support the Nouveau driver project nor do they have any other official driver alternative to their binary blob, so they're sort of left maintaining these legacy driver branches. They have released driver updates to their three legacy drivers plus to their stable 275.xx series.
Coming out shortly after the new 280.11 driver beta were these three releases. The 280.xx driver branch is currently their experimental work, but to the 275.xx stable series they put out the 275.21 build. The NVIDIA 275.21 driver restores the release splash screen, fixes bugs in NVIDIA-Settings, adds new X configuration options pertaining to 3D Vision, and has other fixes related to NVIDIA 3D Vision technology.
Changed in their legacy drivers are just minor maintenance work. To their first legacy driver, the 173 series, is the 173.14.31 legacy. This driver just fixes a bug causing freezes and crashes in KDE4 when KWin desktop effects are enabled on X.Org Server 1.10 and newer. That's it.
In the NVIDIA 96.43.20 legacy driver, for supporting even older hardware, is support for X.Org Server 1.10 along with the 173 series fix.
Lastly, for the very oldest hardware, is the NVIDIA 71.86.15 driver. This driver update just changes the NVIDIA installer to detect the Nouveau kernel module and fail with an appropriate error message, a change that the other drivers have long since carried. The 71.xx.xx driver series has not been exposed to any X.Org Server 1.10 support.