Thank you for your response.
Righ, I’ve looked out for that and do not see it.
Exhibit A:
Are you on android or iOS? I’m on iOS, and the automatically perform…sunset choice is right beneath the “at a certain time.”
Maybe @tgauchat could check, I know he uses android.
OK, I just checked, and the official support article definitely says you should be able to do it automatically at sunset:
https://support.smartthings.com/hc/en-us/articles/205956706-How-to-create-automations-with-Routines
As you can see, there are lots of different options and settings that offer full customization, including triggering devices to perform an action when someone arrives, when everyone leaves, when motion is detected, when a door opens or closes, at a certain time, at sunrise or sunset, only during certain Modes, etc.
So since you aren’t seeing that, I would contact support. You may be right, it may be that somehow it was dropped from a recent app version, but it doesn’t look like it was supposed to be dropped.
Sorry for the confusion earlier, I didn’t realize it might be different in different versions of the app, but that has sometimes happened in the past. 
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It’s there now.
There’s a real chance what I was recommended to do in order to fix one problem trying to setup my first piston in CoRE, actually fixed this problem. If you notice in that post https://community.smartthings.com/t/core-get-peer-assistance-here-with-setting-up-pistons/50187/8371?u=thing1nthing2, I had to re-submit or commit my location again ( for whatever reason to probably update something else on ST’s cloud I suppose) for me to see this option under the routines.
Come to think of it. I think this location might have impacted my other routines for not working as I expected it to work; from a theoretical standpoint to practical use.
It just so happens I caught this post after looking for an update once my CoRE issue was resolved.
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Sunset is location-dependent, so that would make sense, but I’ve never heard of that issue before. Interesting. 
I set night mode like most above by having things quiet down when my living room Motion Sensor has no motion for 20mins after 10:30pm. But what if my wife ( or me ) is still out after ‘things quiet down’
One of us is in and things quiet down and so ST switches to night mode. This has things like my front door sensor etc armed. Then the wife comes home…