The only 9 series card that can possibly support HDMI 2.0a is 950, every other 9 series card predates the date that the HDMI 2.0a spec was finalised, so if 950 doesn't support HDMI 2.0a (which I don't think it does, at least not according to nVidia), then no GPU in the world currently supports HDMI 2.0a.
9 series card are also the only cards that even support HDMI 2.0, none of the AMD cards to IIRC.
It uses the exact same bandwidth and HDCP version as HDMI 2.0, and it just trades 4:2:0 Chroma subsampling to free up enough bandwidth for HDR.
Don't believe me? Have a look at how many older TVs will get a software update to 2.0a. Those TVs have been on sale exactly as long as the GTX 960 has, so the hardware should be of the exact same spec.
I'm sure Nvidia will announce support for HDMI 2.0a when the 4k Blu-Ray players are finally available. No point in announcing a feature and making a marketing push unless there's something you can hook it to, right?