It will disappear from your list of devices under System in the Sonos app after a few days (usually 72 hours) of it not being detected on your network.

Did it disappear ? :)

That’s from the controller. From the Sonos website that shows your devices that are connected to your Sonos “account”, it requires someone else to register the device to their account. 

I’ve got several decommissioned devices (that were recycled) long ago, mostly BRIDGEs, that still show up on the Sonos website as being registered to me. 

As near as I can tell, that database is locked, and the only way it can be modified is by someone changing the pointer externally. To the best of my knowledge, no one inside Sonos, including customer service, can touch that data. They can see it, just not modify it. Which makes some sense to me, having managed other databases in my time. I think of it more as a “this is a list of purchased and registered items” rather than anything resembling “this is what you’re using currently” list.