Perform Goodbye when SOMEONE leaves?

They are definitely making changes for the V3 app. See the following FAQ:

FAQ: Why can’t I use Geopresence or an Automation to set the alarm state in the new (V3) app? (Updated 1 November 2019–now you can!)

But the last I heard, that only applied to security features. So locks but not lights. I don’t know if they’re changing more than that.

As far as alternative means to presence, there’s an FAQ for that, but it applies only to the classic V2 app. It doesn’t let you get around the V3 app restrictions.