An update, to i think is the "final" way to have pi4 b as i2c slave only,
kernel i2c support (raspi-config) is needed only for test with i2cdetect, and can be disabled without losing slave functionality, pigpio only will take care of slave
but must keep all 4 wires in, detaching "master" gpio will stop arduino trasmitting around one minute later (i use a logic level shifter)
I suspect it's working this way because the RPi I2C bus on pins 3 & 5 provide 3.3V pull-ups for SDA & SCL. Did you disable the 5V pull-ups on the Arduino as described at https://forum.arduino.cc/t/i2c-pull-up- ... s/286734/7 ?