Thanks, @homeagain, very much for your reply and background…I had also continued to search and found some of that info in some other posts here as well, but very helpful to see your summary/experience.
I returned the dimmer switch that was trying to join insecurely (though I had ordered a nomal switch, a dimmer switch was erroneously included in a normal switch box - Amazon Warehouse used deal). I received the replacement Honeywell Zwave Plus regular switch today, and it installed and completed the S2 security addition normally (though a new process to me - been quite a while since I’ve added any new switches).
The one problem remaining is that it appears that when you add a switch that does not have S2 security, there is no path in the device add path in the new ST app that allows the device add to complete. I can only back out of the device add flow, and I end up w/the insecure device not showing up in my device list.
When you installed insecure devices in the new ST app, were you able to complete the addition of the insecure device(s)? As I noted previously, when you hit the Skip button in the screen shot above there is no way to finish adding/naming the device. Something seems broken here in the new SmartThings app w/adding insecure devices.
EDIT: I did eventually find the first device I added recently that added insecurely (a Zwave plug) in my device list finally - it added as a generic “Z wave device.” I never did find the GE Dimmer switch that added insecurely over two days of looking for it in my list, including looking for it adding as a generic device. Though I could not find it in my device list, I was able to remove it via normal Z Wave exclusion, which was odd. I removed it yesterday when the new swtich arrived. So that second device (GE Dimmer Switch) never seemed to show up in my device list, but the first one (Eva Logik plug) did.