Even if you had reset them they would still be there.  There is no way to remove them but it does no harm.  If and when the speakers are resold and installed on someone else’s system, they will vanish from your account.

Oh, that’s annoying because they show up in my other apps like Yonomi, Google Home...

That doesn’t make sense to me.  I can’t see any way that what is shown in your Sonos online account would be relevant.  Are you sure you don’t need to reboot or refresh these other apps, or delete the Symphonisk speakers from them?

Unfortunately the apps only display devices from the Sonos account. There’s no way to edit/remove devices on any of those apps.

How do think that what appears in your online Sonos account as registered devices can be picked up by systems on your home network?

I always thought it was UPnP but apparently not

The data that is on the Sonos website is not a list of items that are in use, it is a list of items that you have registered as ever being part of your system (purchased), and never registered to someone else’s account.

The only way to remove items from that list is to have someone else register that device to their account.

I’ve got several retired devices, from BRIDGEs to an old PLAY:5 Gen 1 I traded in.

I’d suggest that these other apps that you mention shouldn’t be using that data as what is currently in use, since there is no real relationship between those two concepts. 

This is admin information held on a website with password protection. It is not part of your system.  I am struggling to understand how anything on your network could access that information. 

That’s a great point. I wonder if these other apps are creating their own stored data, and not accessing that data from Sonos?