Yes, the SPIx_RDY signal is necessary for SPI slave boot.
In SPI slave boot, the SPIx_RDY output is requires a pulling resistor(10K ohm). The boot code requires the SPIx_RDY signal function as active-low. The host is only permitted to transfer data when SPIx_RDY is in the active state. This functionality allows the processor to hold off the host while the processor is reset or executing the pre-boot and processor initialization sequences.