The "Security deposit" is no longer available in the section "Policies and Rules" per Airbnb guide. Yet, the Hosts who have more than one listing are still able to set up their security deposit in the section "Listings".

How to add Airbnb security deposit.
How to add Airbnb security deposit.

Also, you may set it up using the App on an iOS device (iPhone, etc). It's NOT available in the App on Android devices.

Unable to add or change Airbnb security deposit?
Unable to add or change Airbnb security deposit?