Can the VBAT pin be floating if the RTC function is not required?

[a_ziliu]

No. The VBAT pin is used to supply power to a RTC module, as shown below. When the VBaT pin is floating, the RtC module condition is unknown and will generate power to affect the standby power performance.
In this case, it is recommended to connect the VBAT with VDD to prevent the power consumption of the RTC module from being increased.