ScruffR is correct, FLASH_Unlock is undocumented. It did previously work by chance because of the STM32 peripheral library.
However, I believe what happened is the flash writing code was modified to add a mutex to prevent different parts of the system firmware from writing to the flash at the same time and corrupting it. However, __flash_acquire is not exposed through the HAL to user firmware, which is why it no longer links.
I think it’s unlikely that this will be changed, since it was never really intended for user firmware to be able to write to flash, but I could be wrong about that.