When I leave one side open (e.g. pins 1,2,3,4 or 5,6,7,8 not connected),
can I assume that SDA and SCL of the other side are off (high impedance)?
That'c correct. They will be HiZ when the opposite side is unpowered.
Just be aware, it is possible to power up side 1 through the ESD diodes on SDA1 and SCL1. So if there is no power at VDD1, but there is a high on the bus, side 1 can power up.
If you want to turn off power to the ADuM1250 on a live bus, use side 2. It can't be powered up through SDA2 and SCL2.
thank you, that helps. My application is this: SCL1 and SCK1 have a pullup to VDD1 and all 4 pins of side 1 go to a cable connector. I want to make sure that, when there's no cable or no connection at the other end of the cable, then SCL2 and SCK2 must be HiZ. According to your answer this is the case. Due to the pullups there's no powering via ESD diodes.
If you are running I2C off board on a cable, using a pair of LTC4331 devices may be better suited for the application.