My understanding is that we only support SPI Slave on specific platforms, such as the 4390.
You may want to also run this past the module partner you are working with as well (Murata, Inventek, LSR, etc.)
The question asked was about I2C Slave support not SPI slave support. I too am interested in I2C slave support. Is there an plans to provide I2C slave API support in upcoming versions of WICED SDK 5?