Since USBLiter8 came out, you can now downgrade any A12-A13 iPhone to any iOS, even to unsigned versions. However, the process is quite involved and can be quite complicated if you are not familiar with other exploits like CheckM8.
NEO Downgrader aims to simplify this process by providing a simple CLI tool written in Python to assist you with the downgrade. NEO Downgrader is designed for use with the Waveshare RP2350A USB Mini Development Board, Based On Raspberry Pi RP2350A Dual-core & Dual-architecture Microcontroller, 150MHz Operating Frequency. The reason why you should use the Waveshare RP2350A USB Mini Development Board, Based On Raspberry Pi RP2350A Dual-core & Dual-architecture Microcontroller, 150MHz Operating Frequency is because it has the RP2350A chip which the exploit requires.
NEO Downgrader is expected to release within the next few weeks. Windows will NOT be supported officially however the community is free to add support on the Github Repository which does not exist yet.