There are details on NVIDIA's plans on their website, where the graphics card maker announces they will discontinue new driver support for legacy Direct X 10-era products after April 1, 2016 (thanks Maximum PC via HARDOCP). Word is the upcoming 340 drivers will be the last series to specifically support older cards such as the 9800 GTX:

After Release 340, any subsequent Windows driver release starting with Release 343 will cease to support the products listed in this section.

The Release 340 drivers will continue to support these products until April 1, 2016, and the NVIDIA support team will continue to address driver issues for these products in driver branches up to and including Release 340. However, future driver enhancements and optimizations in driver releases after Release 340 will not support these products.