The datasheet mentions that the SCL0 and SDA0 pins require external pull-ups regardless of PINMODE / IOCONF / PINSEL configuration. But on SCL1, SDA1, SCL2, and SDA2 look to be able to use an internal pull-up ?

[lpcware-support]

I2C0 pins are open-drain so they require external pull-ups.

I2C1 and I2C2 pins are not open-drain pins. The internal pull-ups could be enabled to pull the signals high when they are not driven low, but these pull-ups are weak compared to external pull-ups and are not recommended.