I think that if you take a close look at the LCD serial backpack board that you have, you’ll see that the pins labelled D0, D1, D2 & D4 aren’t actually connected to anything - they don’t have any PCB traces connected to them and no connections to the pins on the PCF8574
This schematic confirms that:
This means that at most you only have 4 outputs rather than 8. You could daisy-chain multiple ones together of course, up to 8 on one I2C bus, provided you gave each one a unique address.
The default address (with jumpers A0, A1 and A2 disconnected) is 0x27
More info here…
Pete.