WebCoRE does not work with phone (mobile) presence sensors in the new app. Nor can it control SHM or Samsung appliances in the new app.
Edited my post to reflect phone presence sensor since it led to confusion.
WebCoRE does work with Samsung arrival sensors - the device you can purchase.
You can use presence to turn on and off a virtual switch.
Some folks feel it’s mandatory to always be on the latest version of an app. Regardless of consequence.
I received notification that the old app was being sunset and that I needed to move to the new one. If that’s not the case, I’ll go back, it was a much better experience.
Not anytime soon. They have a lot of work pending on the new app before it’s ready for prime time.
I believe the staff has mentioned that the Classic app is no longer being worked upon (except for critical security updates). This by no means says that it’s ready for sunset, if this were to happen anytime soon it would create total havoc as the new app (apart from not being completely stable and still slow) doesn’t support installing custom apps/devices as yet and is missing many critical features (such as routines, SHM control, lock/presence automations) which folks still use very happily on the Classic app.
So yeah, go ahead and use the Classic app and you won’t be missing out on much.
There’s no guarantee of when it will be. If they shut down the classic app tomorrow, there would be a group of power users (the majority of members of this forum) who would be really mad, but the majority of smartthings customers wouldn’t care. (they have told us many times that the majority of their customers have 15 or fewer devices and never use any custom code.)
Look at what Google is doing with the “works with nest“ program. My guess is Samsung is going to be watching customer reaction to that pretty closely. If google gets through that process without a huge PR hit, Samsung may be more tempted to drop the classic app sooner.
The untold story of Google I/O 2019 - The irresponsible attempt to kill "Works with Nest"
There are now quite a few devices that work with the new V3 app but don’t work with classic.
Sunset is coming, we just don’t know how fast the planet is spinning. 
You’re right in principle but in reality in ST’s world things are measured in years 
I use webcore to do it. My rule is:
So for it to be triggered when I’m not at the stoop would require someone to open my storm door when I have just returned to the geofence but I’m not yet at my door. It doesn’t unlock if I pass through, and if I don’t leave the geofence (like if I’m at the neighbor’s or the bar on the corner) it won’t unlock. It also won’t unlock a second time (like if the UPS guy comes and opens my storm door after I just came home).