Sorry for replying, but I wanted to get your opinion on this. Looking at the Portenta H7 schematics, I'm connecting I2C either through GPIO pins (JDIGITAL) or through J5 via 5-PIN QWIIC Cable. It goes to I2C3 on the microcontroller directly. Now, according to the datasheet of the STM32H747, the pins for the third I2C port (I2C3) in the link below are classified as I/O pins (Table 7). Furthermore, looking at the electrical characteristics, I/O pins are capable of tolerating 5.5 volts (assuming that the IC is running at 3.3 volts for VDD, VDDA, and VDD33USB). Thus, despite the Portenta operating at 3.3 volts, it should be able to tolerate 5 volts for I2C, correct? I mean, thinking about it, if it can't tolerate 5 volts, why would their additional I2C connector use 5 volts on the pin out instead of 3.3 volts? It's been established that you can connect a QWIIC cable from the Portenta to the sensor and the Portenta doesn't smoke or anything.
https://www.st.com/resource/en/datasheet/stm32h747ag.pdf
Also, to add on, there's an application note for STM32 processors (that I found on the product page of the STM32H747) showcasing how the GPIOs are 5-volt tolerant. The I2C example connection shows them using 5 volts while the processor runs at 3.3 volts. So, does all this mean that I am safe?