There’s a point of diminishing returns as the sample rate goes up. With DSD a higher sample rate provides more S/N but it comes with higher jitter. Somewhere between double rate and quad rate is the optimum and higher than quad rate is getting worse not better, in addition to taking twice the disk space/bandwidth as the rate increases.
The DS was designed for double rate DSD (at a time when no double rate DSD was commercially available) and in fact no matter what the input it ultimately converts it to double rate DSD. There’s a remote chance that a future software upgrade will accept quad rate and output double rate (or possibly even quad rate) to the hardware. It’s not on the schedule at this point and won’t be added if it makes any of the current formats sound worse.
Not that I’ve made a study of it, but I don’t know of any sites that in general don’t offer a single rate DSD version of anything they offer in higher resolutions and similarly I don’t know of any sites that don’t offer double rate DSD for anything they also provide in quad rate or higher. I’m sure that some one off tracks, especially for high rate demos only come in limited versions, but in general there aren’t sites that only sell PCM in, say, 192k without also offering it in 96k and 44.1k, there’s no reason to expect that to be different in DSD land.