FDK AAC

Info

FFMPEG Audio Encoder fully supports this tool and will continue to do so

As of v4.0 I can no longer pack fdkaac.exe with the program

You technically need to build the executable yourself or use other means to get the executable file. If you know where to look you can easily obtain it. Once obtained, add the path to it in the settings window, or simply add it into it's app folder, restart the GUI, and it will automatically detect fdkaac.exe and re-enable the encoder in the encoder menu