Powering down unused physical RAM is absolutely a thing on some systems. For one thing, it's required if you ever want to support physical memory hotplug. The real issue however is that the gain from not doing DRAM refresh is clearly negligible: it's no more than the difference between putting a computer to sleep (ACPI S3), or putting a phone to sleep in airplane mode - and powering it off.

And you're saying Apple is doing that on the iPhone?