I could not find anything in the datasheet.
What is the best practice for unused pins for SC16IS752?
I currently do not need TXB, RXB, /RTSB, /CTSB and GPIO0 to GPIO7.
(I know a lot of unused pins, I am using this IC for another project already which makes it easier.)
I think it is a general guide for unused pins that the output pin can be float(No Connection), the input pin can be connected to external pull-up or pull-down resistor.
Hope it can help you