You don’t see it in x86 I think, but I believe ARM mobile SoCs can completely turn off banks that are unused. I haven’t followed to see if anyone actually does this yet, but I recall hearing about attempts to turn off unused RAM when in sleep (when awake, the RAM refresh cost is negligible). To turn off the RAM implies that you might want to prefer to place cache data in the banks that you will want to turn off so that you can just drop caches on sleep quickly. Of course, whether that’s actually beneficial in practice is hard to say - some caching you want to be persistent as you’ll need it on wake anyway.
Outside cases where you want to optimize for sleep performance, you probably don’t want to do this though because NUMA systems have conflicting requirements for storage (but thankfully for now the systems with NUMA and the systems that benefit from turning off RAM refresh when sleeping has 0 overlap). Consumer desktops are probably not worth doing this on and laptops probably isn’t a huge difference because of battery size / usage patterns.
You're probably thinking of DDR self-refresh which lets the entire CPU (including the memory controller) power off without losing data when the device is in sleep mode. I've never seen hardware that partially powers off memory the way you described.
My memory could be faulty, but I didn’t think it was self refresh which has been quite common for a long time. Maybe it’s possible the OS vendors never got the idea working on the software side and abandoned it / in practice you could never arrange to have a good likelihood of having an entire bank of memory unused. You could estimate the benefit with SW experiments cheaply before ever going down the path of building the required HW support.
Outside cases where you want to optimize for sleep performance, you probably don’t want to do this though because NUMA systems have conflicting requirements for storage (but thankfully for now the systems with NUMA and the systems that benefit from turning off RAM refresh when sleeping has 0 overlap). Consumer desktops are probably not worth doing this on and laptops probably isn’t a huge difference because of battery size / usage patterns.