Why are MOSFETs used on the battery gauge lines?
previous models didn’t have mosfets on the gauge lines so I’m wondering why they’re used now


The older iPhone 6s does not appear to use I²C signals for its battery gauge - those signals mention "SWI" on the partial schematic supplied.

The purpose of those MOSFETs in the iPhone 10 is explained on the partial schematic :-) It says:


  
    Gas gauge I2C level translator
  


Q3200 is the level translator for the SDA signal and Q3201 is for the SCL signal. For these, the device shown is the Rohm RV1C002UN MOSFET.

From the partial schematic, these signals on the iPhone 10 are 1.8V I²C signals, yet the gas gauge chip contained as part of the battery "module" is using a higher voltage (I don't know what voltage) for its I²C pull-ups. It appears that the relevant 1.8V I²C IC in the phone is unable to accept a higher voltage on the I²C signals, hence the need for level translation.

Using MOSFETs in this way, is a common, cheap, way to perform level translation for relatively slow signals. See NXP Application Note AN10441: Level shifting techniques in I²C-bus design. This technique is also explained in more detail in the old Philips Application Note AN97055: Bi-directional level shifter for I²C-bus and other systems.