GPG Mail not in Manage Plug-ins list after installation or doesn't remain active
What you are seeing is a bug which occurs if during the installation of an older GPG Suite SIP was deactivated, but is now activated.
To fix this problem:
- Close Mail.app
- Open System Preferences › Security › Privacy
- Enable Terminal under Full Disk Access
- Restart Terminal when prompted
Run the following command:
sudo rm -rf ~/Library/Containers/com.apple.mail/Data/DataVaults
If you are seeing the error "Operation not permitted", you will have to temporarily disable SIP and repeat step 5.