I contacted Wink support today. A possible workaround would be to enroll the Nimbus as a Pivot Power Genius in the Wink app.
The Quirky + GE Pivot Power Genius device is also powered by Electric Imp. Because the Pivot is also enrolled with the Wink app and is still supported, it may be able to enroll a Nimbus since the pairing method is very similar.
I just ordered a used Nimbus off eBay to try it out. Wink support told me it’s not guaranteed to work, but they shared this method with me because others have reported success with it.