I2C LCD support has not been integrated with the latest firmware yet. The pull request here:
https://github.com/OpenSprinkler/OpenSprinkler-Firmware/pull/8
implements the I2C LCD for OSPi/OSBo, and it uses a pretty common I2C LCD that you can easily source on eBay or Amazon. It’s based on PCF8574 I/O expander. It is NOT compatible with MCP23017.
Honestly if LCD and buttons are important for you, you should probably consider the fully assembled OpenSprinkler 2.3. The unified firmware works for both hardware.