My recent Samsung has the same dumb bug. Turning off UPnP on the Sonos player would render it useless, it is the protocol used for everything. The real fix needs to be in the TV - either detect DLNA support properly, or give the option to remove errant devices from the list. My Samsung has neither. However I hadn’t thought of putting the TV on a separate subnet, that is trivial to do with my router.

The problem with multiple subnets is that you will inevitably encounter strange issues and it takes a lot of time and effort to debug. I don’t understand why you claim that turning off UPnP would render the device useless? There could be legitimate cases where you just don’t want it running. And you can control exposed ports on your router yourself and it generally does not affect outgoing unless you set your firewall up like that. However, I’m not sure whether it’s UPnP or DLNA that causes the device to appear as a separate network entity within the TV’s software. Bottom line, LG’s smark speakers don’t do that, so there clearly is a way.

Sonos relies upon UPnP to communicate between your different devices, it is fundamental to the system operation.

I don't know enough about DLNA to explain why this happens but it appears to be fairly common that DLNA devices incorrectly detect Sonos devices, My Panasonic HTB sees Sonos as a DLNA server and the smartphone app  BubbleUPnp (amongst others) sees them as both Servers and renderers, they work quite well as renderers 

Happy to report that switching my TV to a different wireless subnet (the one set up for my kids) made my Sonos devices vanish from my Samsung TV Input menu. Can’t believe it took me this long to realize this fix. Happy wife, happy life.

So to solve this annoying issue, you have to be a network geek…

I’m not, so my first question is - what functionality would I loose if Sonos gave me the option to switch uPnP/DLNA server functions off?

I can’t see any situation where Sonos will be the server of anything?

You would lose every feature. Each speaker exposes around sixteen UPnP services over the network, and all are needed for the controller to do anything with your speakers. Setting up a Guest network (for your TV) doesn’t require you to be a network geek, most routers offer it as a feature.

I connected my LG TV to my guest network to avoid this. Of course, I can’t reach content stored on my home network via the TV but this isn’t an issue for me.