For the TXB family, unused I/O ports are recommended to be tied to GND through a weak pulldown resistors (>=100kohm).
For TXS devices, the internal 10kohm pull-ups resistors will ensure that the I/O ports are never floating and hence can be left disconnected.
For LSF devices, the unused I/O ports can be left disconnected as they are passive switch type translators.
For AVC/LVC devices, the unused input and output ports are recommended to be tied to gnd.
For AXC/LXC devices, the unused input and output ports can be left unconnected as there are very weak pulldown resistors internally. This helps in avoiding unknown voltages at the pins thus reducing the power consumption due to floating voltage levels.