I recently purchased and installed a Leviton Decora Smart Dimmer 2nd Gen WiFi wall switch. I have had good luck with Leviton before. The switch works fine manually, with the Leviton app, with Amazon Alexa, and with Google Home. After some difficulty I can add it to my Smartthings home and assign it to a room, but it shows as offline even though everywhere else it is showing as online and works.
I wonder if the 2nd Gen Leviton devices are not yet in sync with Smartthings, or vice versa?
It might be this:
I have now corrected my post, but one important item I left out is that my device is WiFi, not Z-Wave.
Yes, I saw that. It has affected virtual Devices, zigbee Devices, zwave Devices… yours was the first Wi-Fi device I saw reported, but it’s obviously an issue on the platform side. 
Interestingly I just looked at my Smartthings app, and the device I referenced above is now considered to be online. I did nothing to make that happen, so I will be keeping an eye on it to see if it breaks by itself again as well. Also of interest is the fact is that the History is showing all of its off and on states even during the time it was being shown as offline.
Many people are reporting that their devices are back online since the notice was posted to the status page that it has been resolved.