Read the FAQ: they are not planning to add the functionality. They made a change in design philosophy for security features with the V3 app, perhaps influenced by their partnership with ADT. You can no longer use the app to automate security features like unlocking a smart lock or changing the SmartThings home monitor armed state.
This isn’t an unusual design philosophy: with the exception of abode, most residential security systems have similar restrictions. You can get a reminder to arm/disarm, but you have to consciously make the change each time.
( I follow these issues closely because I myself am quadriparetic, and the whole reason I got home automation to begin with was to have a hands-free option for unlocking the front door. The removal of this option is a very big deal for me. )