Is RAM always readable
The part you're using has an SMBus interface, so the question is not one of whether RAM is readable, but rather whether or not the device will let you read specific addresses.

The answer is that it's entirely up to the design of the specific device, and no general rule applies here. If the datasheet does not list the address as readable, the device may or may not return it if queried - and if it does, that could change at any time with newer revisions.