I notice that 4-wire designs are absent from AN70707 PCB Guidelines. Can you please verify the feasibility of my proposed 4-wire microSD connections? In particular, that I am free to ignore signal WP, which does not exist on my microSD connector. VIO2 working voltage is 3.3V. What considerations are required to lower this voltage to 1.8V?
This schematics looks fine. There isn't any special consideration for 1.8V. It is enough if you connect the VIO2 to 1.8V.
Make sure the firmware has io_cfg.s0Mode = CY_U3P_SPORT_4BIT in CyU3PDeviceIOConfigure
and lowVoltage = CyTrue in CyU3PSibSetIntfParams API