Nvidia typically stops supporting it's older graphics cards cause simply put, it's not worth the time to optimize for every single GPU out there.
Plus, when there have been so many graphics drivers for an older GPU, those drivers are pretty mature so you don't really need new ones. Unless your trying to play the latest titles which wouldn't works since the GT 610 can't play the latest titles.
Technically, Fermi is still supported but in "legacy support phase" until January 2019. This means there will be no more updates except for critical security bugs until then, and after that you are on your own. You can read their own explanation why here. In short: not worth their time.
As for the "maturity" of the drivers, it was just last year that Fermi and Kepler cards would never wake or BSOD on waking from S3 sleep, so for almost an entire year the drivers simply removed the Hybrid Sleep option in Control Panel's Power Options unless the BIOS was set to S1 sleep (which leaves the RAM and all fans running at full speed). It's fixed now but just goes to show that optimizations for newer cards can have buggy side effects on older ones, even those still officially "supported."
That's still better than Intel, who are infamous for regularly discontinuing IGP drivers after two years. Fermi came out back in 2010