The KeyStone devices I2C pins are open drain IOs, and require pull-up resistors to DVDD18 rail. The recommended pull-up resistor is 4.7KΩ on SCL and SDA lines.
If the I2C signals are not used, it is recommended that the SDA and SCL pins be pulled up to 1.8 V (DVDD18). These pins can be left floating, but this will result in a slight increase in power consumption due to increased leakage.
The 1.8 V I2C pins are not tolerant to higher voltages. The PCA9306 dual bidirectional I2C level shifter is recommended when connecting to higher voltage I2C busses like 2.5 V or 3.3 V.