From the datasheet, section 4.1 : "In case the I²C interface shall be used, a direct electrical connection between VDDIO supply and the PS pin is needed in order to ensure reliable protocol selection. For SPI interface mode the PS pin must be directly connected to GNDIO."
As you can see, it is definitely not recommended to leave the pin floating. But indeed no-motion and any-motion interrupts are fairly simple, and the BMA253/280 have these interrupts implemented directly on the ASIC itself.
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