TMS320F28388D: why does the requirements that pullup must be enabled only be asked for 176 PTP packages?

Hi Yale, thank you for the question.  Unused device pins should always follow the guidance provided in the device datasheet (see Connections for Unused Pins).  The purpose of the comment you highlight is to ensure unbonded pins in the smaller package are not left floating.  I hope this clarifies your concern.

Regards, Joe