I2C Slave support has been added to ESP32 Arduino Framework
This library provided ESP32 Slave functionality as a workaround for the lack of support on Arduino core and on ESP-IDF, as pointed by issue #118 in 2017. On October 14, 2021, that issue was finally closed by the pull request #5746, and so this library is not needed anymore.
See README_old.md for details about how the workaround was implemented.