Do STC8 microcontrollers have internal pull-up resistors?

Yes, STC8 microcontrollers have internal pull-up resistors which can be enabled individually. They have a value of 4.1kOhm, lower than many other microcontroller pullup resistors. Due to manufacturing tolerance, the pullup resistor may vary between 3kOhm and 5kOhm.

Source: STC8G1 datasheet, section 9.1.3 (value) & 9.3.5 (tolerances)