https://www.youtube.com/watch?v=-GTpBMPjjFc is a good overview of what's up there so far, and what's coming as they really try to scale the technology.
Bandwidth and processing are substantial bottlenecks with SAR; Only targeted and stationary applications have been broadly useful so far, and more focus has been put on planes than satellites for this. SAR is not as simple as taking a static image with a fixed resolution, your sensing window has got a target velocity and distance in mind and the antenna and processing needs to be tuned for that.
I would think that medium and high orbit optical tracking (daytime, cloudless sky) is probably used, because with video you can reasonably track subpixel targets if they're high contrast, without a lot of data transmission requirements.
I'm not sure why you assume this, this is factually incorrect. Satellite based SAR has been successfully used for civilian ship detection applications (traffic management, illegal fishing, smuggling detection, etc) for over three decades. I am sure its military use goes back much further.
> SAR is not as simple as taking a static image with a fixed resolution, your sensing window has got a target velocity and distance in mind and the antenna and processing needs to be tuned for that.
No? SAR satellites take thousands of SAR images of stationary scenes every day. It's true that object motion in the scene introduces artifacts, specifically displacement from true position - this is often called the "train off track" phenomenon, as a train moving at speed when viewed with SAR from the right angle will look like it's driving through the adjacent field rather than on the track. However, this isn't a significant problem, and can actually be useful in some situations (eg: looking at how far a ship is deflected from its wake to estimate its speed).