The AAC export option strangely vanished in the "M4A (AAC) Files ( FFmpeg)" Format option (other options OK)

[Trebor]

Export Audio - Formats missing?

Go to Edit → Preferences → Modules. Depending on your version of Audacity you may see several mod-xxx boxes for various audio formats. Enable them all. Or you can delete your audacity.cfg file and when you re-start Audacity it should build a new-clean one. This page tells you where to find it. When you un-install it keeps the audacity.cfg file. Then when you re-install it uses the old settings by default. (I think it asks a question about that.)