Extend Catalina's privacy protection to other folders In MacOS Catalina apps need to ask for explicit permission to access certain user folders (such as ~/Documents and ~/Downloads).

Is it possible to extend this protection to other folders (e.g. a custom user-created folder ~/foo) without moving it inside the Documents folder?
No.

The restricted file locations are detailed here PrivacyPreferencesPolicyControl.Services and you can't change them.

You would have to move ~/foo to Documents, Downloads, an external drive, or another restricted location.