Amid Evil recently received an update via Galaxy. Before then, we had an option to start the game with RTX options, but after this update said option seems to have been removed. What happened?

I don't have the game here, but assuming there is no preconfigured launch option on Galaxy anymore, you could create a new launch option, add '-DX12" to the shortcut and see if it enables it.

The options are all ingame. The "R.T." section is disabled if the game doesn't run in DX12 mode (that's what the ingame documentation says).
So restart the game in DX12 mode by adding -dx12 to the command line. After that the options are active and can be enabled.

thanks!